I think that Theodore was heartbroken and twisted because he probably could never marry her. She was a slave to his brother and a tool for him. He probably couldn't marry her because her blood would've been considered tainted.
Theodore was a strategist, so he probably gathered all of the people whom he knew would be against her, and betrayed her giving her an excuse to kill him and everyone else who looked down on her and would not support her claim to the throne.
Maybe he believed that she couldn't be truly free if the people who oppressed her were still alive?