Sometimes, a nutter is just a nutter, even when he's a homicidal nutter.
In the case of Jake Davison, the Plymouth killer who murdered five people, then himself, the indications are that he was a sad, bitter, angry man with a grudge against society in general and women in particular. He didn't have a girlfriend, and, like every other sad case nowadays, from cannibals to neo-Nazis, he found company and a label online. At any other time, we'd be content to call him an evil creep, a sadistic coward or possibly a homicidal loon, and confine ourselves to the valid question of how his shotgun licence was renewed.
Now, because identity looms so large in the culture, and because the internet makes it so easy for likeminded lunatics to find each other, we find him conveniently categorised as an incel.