Agreed. The writers failed to convey his narrative as well as they could. And it wasn't because his heel turn just came out of nowhere, they weren't going for subverting expectations in that way. They wanted people to feel sad that he took a dark path, and they didn't hide it. They showed Grace getting off the path, the path that she arguably put him on. He's not a villain that you are supposed to be happy to see die, they wanted you to believe that he could have been redeemed. In the end he was just playing the game that Grace taught him to play, and when he finally won he gets the cliche lighting and maniacal laugh, but a moment later he's crying, and then both laughing and crying. He's not angry to see Grace survive, he's confused and he doesn't survive long enough for us to see what his reaction will be after he finishes processing what just happened. The death of Simon is supposed to be tragic, because he wasn't purely evil. It seems that one of the lessons he needed to learn is to resist Grace's peer pressure. Grace denies responsibility for Simon's actions, but she should accept responsibility for her own. To put it simply it was wrong for Grace to teach him those things, but Simon was wrong to go along with those things.
The problem is that it is difficult to take a character who isn't comically pure evil and make them do evil things and kill him off like he was pure evil and beyond redemption. The way it comes off is like the writers want to have their cake and eat it too. There's too much to unpack, and it wasn't well conveyed. Simon's death is ment to be tragic, not triumph over evil. But it fails to convey that mostly, because they make him out to be too far gone, it isn't clear that he could be redeemed, well not at time that he does anyways. I think if they had him smile or cry or show some kind of emotion when he sees that he didn't kill Grace, the sadness of his death would have hit a lot harder.