If the popularity of Tom Hiddleston's nefarious Loki has not exactly taken Marvel by surprise, it has certainly become increasingly apparent to the Disney-owned studio following the huge success of 2011's Thor and last year's The Avengers.

Hiddleston's portrayal is rivalled only by Robert Downey Jr's Iron Man as an example of a character whose movie stature dwarfs that of his comic-book counterpart. So great is the demand for the character that Loki has already been given a significant and expensive upgrade for the forthcoming Thor sequel, The Dark World. This is not nearly enough, however, for the group of fans that has launched a Change.org petition calling for the prince of Asgard to receive his very own movie.

The "Free Loki" campaign (a reference to the royal baddie's incarceration at the end of The Avengers) cites the reaction to the character at this year's Comic-Con, where Hiddleston's appearance in costume electrified the audience, as evidence that Loki is due a solo outing. "We feel Loki (as portrayed by Tom Hiddleston), due to the astounding following both the character and actor have earned as a result of the Marvel films, more than warrants further exploration of his story," the petition announces.

"We believe such an endeavour would not only be profitable and successful but provide a larger view for general audiences of the incredibly complex and diverse personality a well-known villain of the Marvel universe is gifted with. To explore this darker side of the Thor storyline would be to give credit to a compelling individual, his remarkable portrayer and an appreciative audience that feels a fierce loyalty to both in an unprecedented way. Simply put: he has an army."

At first glance, there don't seem to be too many arguments against such a development. Hiddleston certainly has the presence to carry his own film, as his subtly multi-layered performance in Thor suggests. Marvel is currently giving screen time to characters as barmy as Guardians of the Galaxy's Rocket Raccoon and Groot the talking tree, so why not Thor's adopted evil sibling?

The problem is that the development of a standalone Loki movie would turn Hollywood tradition completely on its head. It is not just comic-book movies that are titled and built around heroes (even when the villains often steal the show); it is the vast majority of feature films, going back to the early days of the form. One usually has to venture into arthouse or horror to find movies centred on evil protagonists.

Superhero and science-fiction movies such as the Wolverine and Riddick sagas have dabbled with antihero leads, but mainstream fantasy film-making has rarely ventured into full-on supervillain territory. Even Anakin Skywalker (later Darth Vader) in the terrible Star Wars prequel trilogy started out as a good guy before those sociopathic tendencies fully erupted.

The only way for Loki to become the protagonist in a new film would be for Marvel to reinvent him as a heroic figure. Something similar is being slipped into Thor: The Dark World, where the villain is freed by Thor to help bring down the even greater menace of Christopher Eccleston's terrifying Malekith. But there are already hints that the slippery princeling will betray his brother's trust.

And there lies the rub. Audiences want to see the trickster god displaying all his treacherous cruelty through ruthless acts of betrayal. Loki is so popular because he makes for a fantastic villain. We may feel a little sorry for him at times, just as it is possible to empathise with Vader's descent into horror through his attraction to the dark side of the Force, but we do not ultimately want him to win. We certainly don't want to see a reformed Loki performing acts of heroism for the greater good. That would be boring, and there are characters more suited to the job.

Of course, there have been supervillain movies of a sort before. Both the hugely successful Despicable Me films fit that bill. But nobody wants to see Hiddleston suddenly adopting a barrage of cute and cuddly little kiddies. It's possible Loki could be exiled – Planet Hulk-style – from the known universe following the events of The Dark World, finding himself in some far-flung galaxy. We might just buy into a movie about that. But his return to wreak havoc – no doubt accompanied by words of wonderfully evil eloquence – would surely be a far more satisfying affair. Give us more movies with Loki, for sure – but movies with Loki as the good guy? Where's the fun in that?