They're being described as the 'new Metallica'. But, after years of making enemies of just about everyone, the band are more cautious about their future

Oli Sykes, at 26 years old, is so slender his shoulders seem to collapse in on his chest. His legs are like drinking straws, his arms so tattooed I can't see where his T-shirt ends and his skin begins. As stage time approaches he is sitting in a tent on a farm in the suburbs of Johannesburg, writing a comic book series on his MacBook. In a few minutes, after a couple of vocal exercises, he will walk the 30 yards from the tent, climb some widely spaced steps to the stage of RAMfest, and find himself the focus of several thousand young South Africans' attention. He will spend the next 50 minutes in perpetual motion, demonstrating why the people at the Sony imprint RCA keep describing his band Bring Me The Horizon as their "new Metallica".

Sykes takes his performance seriously – in fact, he seems extraordinarily driven in all areas of his life (he also has a successful clothing company). He whips the crowd into walls of death and circle pits, bellowing encouragement. He dares them to try to reach the stage. He throws himself around the stage, jumps into the photographers' pit, climbs the monitors, all the time keeping up his barbaric yawl. Later, the band's recently added keyboard player, Jordan Fish, will say that after every show he wonders where Sykes – so quietly spoken, so still – finds the energy.

Outside metal circles, Bring Me The Horizon mean little. Inside, though, they're a big noise, a band getting bigger and bigger, dividing opinions as they go. In the seven years since the release of their debut album, they've shifted styles from metalcore – a genre that matches the speed of hardcore with the pummelling aggression of metal – to the sound of their fourth album Sempiternal, on which the tempos are slower and the vocals carry melodies – though it's all relative – and washes of electronics bathe the guitar riffs. It sounds not unlike Linkin Park, though it's highly possible students of metal will dismiss that as wildly inaccurate.

"It's important not to overhype – that's crucial," says RCA's MD, Colin Barlow, shortly after saying that the signing "is a landmark deal – it's as important as when Sony signed AC/DC or when Metallica was signed to a major", and that the band will be headlining the Download festival within two years.

The band, though, is being more cautious than the label – even to the label's faces. So when the name Metallica was invoked as the deal was signed, Sykes had his reply ready. "I said: 'You're gonna be disappointed, mate.'"

"We're never gonna sell out arenas. If you get that in your hopes, you're only gonna be let down," adds guitarist Lee Malia.

"We appreciate what they're saying," Sykes says. "And what I take from it is they want us to be their new flagship band. At the end of the day, we're not Metallica, we don't sound like Metallica, we don't want to be Metallica, we're not going to be Metallica. We want to be our own mind." At least we're clear on that, then.

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The band formed at college in Sheffield in 2004, just kids who wanted to make noise. Malia met drummer Matt Nicholls, who proposed starting a metal band. "And he said: I know this singer.'" Malia looks over at Sykes. "And from that I ended up talking to you on MSN or something. It started then, cos I used to play him guitar riffs on the internet before I'd even met him."

"I'd never met a proper guitarist," Sykes says. "Lee blew my mind. We'd be talking on MSN and listening to music and he'd start playing guitar, and I'd be like: 'D'you know this song?' He'd say: 'No.' And I'd be: 'How are you playing it?' It boggled my mind. He's the brains, he's technical. I have ideas but I don't have that much actual technical knowhow."

Malia grew up with a dad who loved classic rock, who used to leave a guitar in his son's bedroom in the hope the boy would pick it up. There wasn't a lot of rock in Sykes's home, but he doesn't think his family's been surprised by his choice of career. "I have ADHD and I were a nightmare as a kid. I were always jumping around and screaming anyway, so it didn't surprise them when I started listening to that kind of music. I used to scream along in my bedroom to CDs and my mum were always at the top of the stairs going: 'Will you shut up?'"

Sykes and Malia attribute the change in their music from the roaring-and-riffs of their beginnings to the influence of a US tour they undertook in autumn 2007 with a band called Nights Like These. "I don't know if they exist anymore, but we loved them," Sykes says. "It were kind of hairy, stonery."

"They were, like, groovy," Malia says. "And that encouraged us to have more groove in our music. They were a big influence."

"They toured round in a little Winnebago and they showed me Pink Floyd VHSes in there," Sykes says. "They showed me Live at Pompeii – I think I were really stoned at the time – and I remember thinking it was awesome. I'd listen to how they'd talk about music and what they wanted to do, and their whole attitude towards it was that they didn't give a shit what anyone else thought."

Mind you, RCA might raise an eyebrow, given what happened to Nights Like These. "Before the CD we liked, they were a metalcore band and they were doing pretty well," Malia says. "They came back with this CD and lost all their fans, but they were doing what they wanted."

Change also came with the arrival of Fish, recruited by Sykes from the mournful electronica band Worship to help change up Bring Me the Horizon's sound. On the bus back from RAMfest, his position within the band is mocked by one of their crew: "Oooooh, without me there's no new record! Oooooh, without me there's no Sony!" It all seems to be good humoured, but Fish is in no doubt about his status as the new boy, not least because he's chatty and open whereas the others are, as he puts it, "very Northern" in their approach to casual conversation.

It's received wisdom that metal – or heavy music, if we follow Sykes's insistence that the band are no longer metal, but simply rock – requires of its makers one emotion above all others: anger. Malia dismisses the idea, pointing out he hasn't written a single riff when he's been angry, but Sykes says it's true. "Music connects more when it had miserable lyrics or angry lyrics, because when you're happy you don't really think about anything."

Does that mean he has to think himself angry to write lyrics? "I've always had something going on in my life that's contributed, so I've never had to convince myself to be sad or angry."

Bring Me The Horizon at Sinsphere in 2010. Photograph: Will Ireland/Metal Hammer Maga

On the new album, organised religion is among the targets. "Fuck your faith!" Sykes sings on Crooked Young. "No one's gonna save you." Why? There follows an explanation with the usual suspects all ticked off – religion causes wars, it brainwashes people, there's no need for a church to impose and police a moral code. But there's something else in there, too. "After the last touring cycle, I was in a bad place, a really, really bad place," Sykes says. "I went to get help. I don't know what I was suffering from, to be honest, but the underlying thing was ADHD. It was leading me to self medicate. But one of the first things I was asked to do was believe in God, to accept God into my life and let him help me."

So this was some sort of 12-step programme? Sykes sidesteps the question.

"I didn't understand why I'd need something that in my opinion doesn't exist to get better. I've got a family, I've got friends, I've got a life. I've been fortunate enough not to live in a poverty stricken world. Why should I have to believe in that to get better? When I say: 'Fuck your faith, no one's gonna save you,' it's like – you need to do it on your own. If you're sick, or you're ill, or there's something wrong with you, don't wait on some all powerful force or a miracle to save you, because it's not going to happen. You've got to put the groundwork in yourself."

Self reliance is a recurring theme with Sykes. He's previously said of his own generation: "They're lazy, sit on their arse and think they're doing something." What does he think they should be doing?

Not being hypocrites, first and foremost, he says. "It's people that are preaching and finger-pointing, but they're not doing owt themselves," is how he describes the target of that remark. "I remember calling someone out because I saw them posting about Apple and how Apple was destroying the world. He was tweeting it off his iPhone. It's that whole thing that drives me absolutely mad. You can't talk about Texaco destroying the world if you drive a 4x4. You are contributing to it. In our generation, everyone's been given a platform to say or do whatever they want, and it's such bollocks. If you believe in all that stuff you're saying, then go and fight for it. Don't sit on the internet and post random ill-informed facts that you found off another blog. It's just word vomit."

In their early days, though, Bring Me The Horizon didn't protest or demand action: they embraced the rock'n'roll lifestyle to a fault. "We were just kids," Sykes says, "and all of a sudden people were putting booze down and giving us buyouts. We went mental. We played every night out-of-our-minds drunk. We wanted to have fun and party with everyone. But soon we realised you can't trust everyone. We quickly found out you've got to watch what you say and watch what you do, and we found out the hard way." What does he mean? "Well, you know I got arrested, right?"

In April 2007, Sykes was arrested after a female fan complained he had urinated on her after she refused his advances on a tour bus. It was then alleged that an unidentified member of the band's party threw a bottle at her, hitting her in the face. The case was dropped, but the band became despised. "When we started the band, I didn't imagine so many people could hate us, or that we could be a band that would make so many people angry," Sykes says. "At first it upset us, but we've come to realise it's how the world works."

It changed their relationship with fans, too, making them wary of interaction. And that has created its own problems with people filling the void on social media and impersonating them. Malia had a strange meeting with a teenage girl who insisted he had been corresponding with her on a social network and had demanded she turn up to meet him. Sykes had to talk another fan out of killing himself, after it turned out a gay man who had agreed to marry the fan was not in fact Sykes. And in October 2011, a 20-year-old man called David Russell was jailed for 17-and-a-half years for kidnap and attempted murder after pretending to be Sykes on Facebook, then luring an American fan over to the UK, where he repeatedly stabbed her, hit her in the face with a log and headbutted her.

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Sykes is horrified someone would do that. He's also disturbed the fan was willing to fly over in the first place. "Even if you were talking to someone who wasn't famous, you wouldn't buy a ticket to a different country across the ocean to meet someone you haven't even seen or spoken to over the phone. If you think you're talking to a famous person, and you have no hard proof that you are – you're not. "

After the show at RAMfest, Bring Me The Horizon are back in their bus within half an hour, back at the hotel another half hour later. There is no sign of debauchery – everyone drifts off to their rooms, and the next morning band and crew kill time beside the pool or by visiting the peculiar shopping-centre-cum-casino-cum-Tuscan-village next door. All except Sykes, who never stops working. At 1.30am, a little over three hours after coming off stage, he's emailing the other band members proposed designs for merchandise. The next day he spends in his room, cracking on with the graphic novel. That Metallica comparison? Not so far-fetched, perhaps.

Sempiternal is out now on RCA