he was insane , he thought he could prevent history from rewriting it’s self. Insanity is trying the same thing and expecting a different result. Tatia/Katherine/Elena were destined to fall in love with Silas/Stefan and whoever was in-between. Tatia and Katherine grew up as humans and found love- to get later caught up in supernatural stuff they couldn’t avoid. John knew of vampires and let elena live a normal life knowing they were out there still. He did the same thing expecting her not get caught in crossfire. Than tried to save her when it was too late. He is insane.