I don't think a few shouted lines support any claims that one is "dangerous unstable". Much less fatally so.
I mean, have you hung around in a big city with some of the mentally ill homeless populations? Some of them will absolutely act very erratically and shout threats because they're suffering from mental health issues. Yet I don't see people treating them as deadly threats and gunning them down left and right.
The only argument I'm really seeing here is, "Rittenhouse was a panicky kid and panicked" which isn't exactly a strong defense.