I probably would but I'm not paid and trained not to - the police are.
Jack Cover, the NASA researcher who began developing the Taser in 1969 was a Tom Swift fan so he named his invention after Tom’s fictional rifle. “Thomas A. Swift’s Electric Rifle.”
Must be a hell of a shot to be putting anybody else at risk with the shotgun pointed at his head. Psycho scumbag or not it's still manslaughter at the very least if they tasered him in that situation making them no better than him.

Maybe the coppers finger slipped, could have been an accident.

I'm guessing that Moat probably made it clear at some point that he was about to commit suicide, so a taser was deployed in a last-ditch effort to stop him?
A rubber bullet probably would have been a better option though.
Looks like they had no option he was going to do it anyway.‘Tell the kids I love them, tell Sam I’m sorry,’ he mumbled to an expert negotiator only 20ft away from him, along with rambling vindictive comments about members of his family.
As soon as that message was relayed by radio to the senior commander at the scene, all the officers knew what was likely to happen next – and immediately the snatch squad was ordered into action.