Can someone explain schizophrenia in a way that makes more sense to me?

I'll keep this short. But reading about mass shootings and crime sprees, I often see something like 'he was diagnosed with schizophrenia and heard voices telling him to do it', what does that even mean? Like, do these illnesses also completely remove their sense of morality? Shouldn't they just think 'that's an awful thing to do, why would I do that'. And do they have no idea that they are unwell? Do they think everyone hears voices so they can't recognize that they are unwell?

I realize this comes across as possibly insensitive, but I'm just genuinely trying to wrap my head around the whole 'a voice told me to do it', as a justification/excuse for doing something horrific, like it shouldn't matter if a voice told you, you still choose to follow through.