Seeing those people shouting his name with both hate and anger made Isiah remember the event that ended his first life, and instead of being overwhelmed like he was for the first time, he felt nothing but hate towards those brainwashed individuals.
In the past he used to feel some guilt whenever he got to feel something like that, he feared those volatile emotions due to his past.
Anything related to harming another human being was unacceptable to him, therefore he kept on being a pushover, a people pleaser just to keep his sanity first, then his morals eventually.
The worst fear was to end up like his parents, he tolerated a lot for the sake of that.
But when Isiah saw the way Rivok was controlling the masses with his words, he knew that he hated him and he hated everyone following him too.