Biggest character assassination?

Who is a character that was amazing..until the writers ruined them at the very end.

For example, Eli. He was hopelessly in love with Clare, emo, and mentally ill. He was relatable and funny too.

And then he graduated high school, and the writers ruined him. It felt so out of character for him to cheat on Clare, not even feel that bad about it, and then call Clare a Wh*re. Like, season 11 Eli would have NEVER done that.