As much as I agree that such people are "inhuman", and my first instinct (like many, if not most, other people, I'd imagine) is that these people don't deserve

As much as I agree that such people are "inhuman", and my first instinct (like many, if not most, other people, I'd imagine) is that these people don't deserve to be treated like 'normal' people... they do deserve it. If you act in a violent way against someone who is not actively dangerous (e.g. waving a weapon around), you're not better than they are. We have laws for a reason, and this guy probably should spend the rest of his life in jail, but doing physical harm to him just brings you down to his level. When it's done by our government (e.g. capital punishment), it's just making us all responsible.