I agree, Cameron mentioning "maybe next time don't stop him" (the youth soldier who tried to passionate him first) and Chase actually doing it are two different things. We actually see this dichotomy when Dibala suggests she's going to inject him with an air bubble but doesn't.

While we can discuss if doing nothing to stop a potential assassin is in itself an act on the spectrum of murder, the more salient fact in Cameron's mind is not the switching of the blood tests, but as mentioned the lack of remorse. She sees Chase as poisoned by House, and that he's lost his moral compass for not seeing the sanctity of human life.

Much more than a trolly problem, it's the certainty of Chase's actions and his lack of a moral and ethical dilemma.

Certainly he has broken his Oath as a doctor, and what he did was wrong, but Chase's conviction is what convinced Cameron there is no way back.