The truth is that the murderer or thief's happiness is dependent on his knowledge that he has no ownership of temporal things, and that what is best for him is to make progress in improving what truly is his, his character and his mind. His happiness then is dependent on his virtue, and on promoting the happiness of all, so harming another won't bring him happiness, but it will undermine his efforts at happiness.

So, ignorance, which is of course by definition an absence, is the evil. It isn't something; it's the absence of something.

Is the person who is the victim of the ignorance of the criminal really being harmed?

If the victim is ignorant of what he really is, he will doubtless believe so, but the fact is that he is being trained and what he is experiencing is for his good. (Seneca once wrote that we should pity people who have no trials or tribulations because the gods have not found them worthy of training.) If the victim is killed he should rejoice, for now he will get a brief rest before he will want to return for more training. If he lives, he should rejoice, for he is getting more training.

When I train to fight in tournaments I suffer pain, and bruising, and attacks and assaults, but I go to the gym to become good at fighting and win tournaments, not to have a comfortable time. I understand that the hurts I suffer aren't an evil, but rather a good. Our suffering is because we are ignorant. We are ignorant of the nature of this place and why we are here, and that we didn't come here for a vacation.