The police have been going through a rough patch recently. First they were implicated in the phone-hacking scandal – though they managed to escape most of the blame when we collectively came to the surprising conclusion that it was more serious for tabloid journalists to neglect the public interest than officers of the crown. But while they deflected a lot of that responsibility, their attempts to deflect it over Hillsborough have been catastrophically counterproductive. And as senior officers have been caught dining with Murdochs or maligning the dead, officers on the ground have been getting shot and called plebs. Or not called plebs, depending on who you believe.

Meanwhile the Police Federation's attempts to extract retribution for the disputed p-word, in the form of Andrew Mitchell's sacking, have been roundly slagged off by former Labour minister Chris Mullin, who last week described the organisation as "a bully", "a bunch of headbangers" and "a mighty vested interest that has seen off just about all attempts to reform the least reformed part of the public service". He didn't call them plebs though. Then again, some police have taken to wearing "PC Pleb and Proud" sweatshirts, so perhaps the insult has lost its sting? Maybe they'll soon be sporting "Sergeant Headbanger Will See You Now" riot shields or stab vests with the slogan "You Needn't Try Stabbing This Mighty Vested Interest".

Another accessory which the Police Federation advocates is the Taser. In June it wrote to the prime minister asking for the number of Tasers to be trebled so that every frontline officer could have one. "They need to have the proper equipment to do the job," says Paul Davis, secretary of the Federation's operational policing subcommittee. And officers certainly seem to be getting a lot of use out of them. Just last week, a policeman Tasered an elderly blind man as part of an operation to check whether his white stick was a samurai sword. It wasn't.

Of course, anyone might get rattled by a semi-paralysed blind man slowly tapping his way with a stick towards you. Apart from anything else, it's so spooky. Those are the high-pressure moments when you need the training to kick in. According to guidelines, officers are permitted to use Tasers when they "would be facing violence or threats of violence of such severity that they would need to use force to protect the public, themselves and/ or the subject" and this moment is surely eerie enough to qualify. The whole event would barely be worthy of note if it weren't for the fact that they shot him in the back. But then the gentleman is quite old and has suffered two strokes in the last few years – so the comparatively slow rate at which he was fleeing was probably taken as provocation.

Incidentally, if you do get Tasered by the police, it's advisable to watch your language. As Boris Johnson has pointed out, it's now an offence to swear at a police officer. So, should you incur a public-spirited 50,000-volt warning shot – perhaps for brandishing your pension book in an aggressive manner or because a young PC has mistaken your tartan shopping trolley for a piece of field artillery – don't accidentally shout "Oh fuck!" or you might get sent to prison. Keep it to a "Dash it all, that smarts, constable!" and be on your way. As soon as you can stand.

In the case of Colin Farmer, the suspected samurai, the police have apologised and Chief Superintendent Stuart Williams said: "We have launched an urgent investigation to understand what lessons can be learned." That response demonstrates everything that's wrong with large organisations. In terms of dereliction of duty, I think it's worse than Tasering a blind pensioner. What possible good can this "investigation" do? We know what happened. A police officer, who Colin Farmer described as "an absolute thug with a licence to carry a dangerous weapon", made a brutal and stupid mistake. How can an investigation illuminate the situation further? Will DNA analysis of the stick reveal that it's a sword after all?

All of which feels like an inopportune context for Keith Bristow, the director general of the new National Crime Agency, to request more police powers. He's trying to influence the new communications data bill so that he'll be able to scour Skype and social media networks for wrongdoers. But he's quick to allay the fears of those who call the bill a "snoopers' charter": "I value my privacy, I don't want to be snooped upon," he explains. "That's not what we're talking about here. We're talking about criminals who run organised crime gangs that import drugs… We're talking about predatory paedophiles, we're talking about dangerous people."

Oh, well that's OK then! He's only going to be snooping on criminals. Personally I don't think that's enough – I think criminals should be arrested and charged, not just snooped on. But just as long as the NCA won't accidentally be snooping on anyone who's not definitely a criminal, I can't see the harm in it.

In an ideal world, Keith Bristow would be wasting his breath. When a state law-enforcement agency says "We need more powers", it should carry as little weight as when I say (as I have spent much of the last couple of weeks saying) "I want you to read my book". Not because either statement is insincere: all writers genuinely want people to read their books and all law-enforcement agencies really believe they need more powers. But, when they say that, they should be completely ignored. Not criticised, not accommodated, just disregarded.

The sincerity is beguiling but it's meaningless. "Help us to do our jobs better," the police implore. "We can see the good we could do if you let us." They almost certainly can. But they can't see what it would cost society in lost freedoms. They can't know the consequences of potentially irreversible authoritarian steps.

It's a frightening state of affairs. Those who know most about law enforcement – those who actually do it – are the least qualified to advise on what its rights, powers and funding should be. We have to ignore their cries and trust our instincts. We have to balance our fears of the indefinable, nebulous worlds of crime and terrorism, with the fact that, if we put Tasers in our public servants' hands, at some point they'll use them on us.

David Mitchell's autobiography, Back Story (HarperCollins), is out now