Why Does It Matter That He Believes It or Not?

Most crimes require not just that you do a bad thing, wrongful thing, but the kind of blameworthy intent. So if you have someone who's completely deluded, in a way, the criminal lass says, well, you're just not as responsible as someone who knows you're doing a wrong thing, and you to go ahead and do it anyway. It just seems to be contrary to how we view terrorism,. for example, which usually, or often, has an under girding of religious conviction  that is fully felt, but still leads to evil acts.