Honestly, it is less he understands what it means to be evil, but a grim understanding that 'good' and 'evil' are such easy things to see differently once you know both sides. In some cases, 'monsters' are just oppressed races that want to see what greenery is, and not just sand, dirt, and hell. In this case, despite monsters representing the evil, they can be just as human as humans themselves are... and in some cases, maybe more so since these people simply saw them as EXP bags.