Something vaguely interesting in the wake of recent terrorist attacks.

[Burney]

So.... what happened to your Venn diagram then?

What I'm saying in no way contradicts that. Of course extremism of all sorts will attract those with mental issues who seek the comfort of un-nuanced absolutes. However, the fact that there is crossover on the Venn diagram doesn't mean that everyone - or even a majority - of those involved in extremist activities is mentally ill.
Much of the time the 'mental illness' thing is simply a comfort blanket that means we don't have to face up to the fact that sane men can kiss their loved ones goodbye in the morning and, with a light heart, go on to perform acts of profound barbarity the same day because their faith tells them it's OK.