Loki's reasons

Back before Loki turned evil he lived a happy life protecting those in need. But than something unexpected happened, the most precious thing he had ever known got taken away from him, a young orphan girl who he had saved from thieves and robbers was stolen from her bed while asleep. After that Loki's life took a turn for the worse. He was weak and venerable, and got into the dark forces and in turn got mind controlled by Thanos.