The phrase 'mental illness' has expanded as a category a thousand-fold this millennium. I see no evidence he has any mental illness, other than a particular kind that is fully voluntarily and reversible.

He seems pretty rational.

If you start from premises that are disconnected from reality, you can use perfect rationality to reach insane conclusions.

Are you armchair diagnosing him? He seems pretty sane.

No, I'm using his actions to diagnose his conclusions. The conclusions ("killing this guy on a public street is the right way to deal with this situation") are insane, whether or not he is.