I actually forgot how amazingly written Simon Laurent is. He's not purely evil and he's a tragic character, almost being killed at the age of 11 and possibly having one of his parents die, leaving him with no hope to live in the human world. You can understand the reason he kills so many people regularly; he was raised by Grace to be that way. It's still however very clear that this tragedy doesn't excuse his actions, with him doing horrible things like betraying Tuba and mocking her daughter about it. His tragedy serves as an explanation for his actions, NOT a justification.
Another great part is about how he's irredeemable, but not in the sense that he's completely evil and has zero redeeming qualities. He does not have the mental capacity to change. He knows what he's doing is wrong, but he's too scared to change. He was raised to be this way and he's too egotistical, though mainly broken, to face the fact that what he's doing is wrong. He straight up says 'Why would I ever want to change when I'm always right?!'. He CAN'T change because he doesn't have the mental capacity to do so, and he stays evil up until his death, which is mourned by Grace and treated as a tragedy. He's undoubtedly a bad person, but you can understand why he turned out this way.
TL;DR: Simon's a perfect example of how tragedy can't justify actions and that some people cannot change.