In the comic, the future Barry found out he made so many mistakes in the past and basically ruined the future, so he went back in time & tried to kill his past self. What if it's kinda similar story. He did a lot horrible things for Iris, so the future flash had to kill Iris to prevent all those horrible things he did from ever happening? We've all seen how far Barry would go to save Iris this season. He was willing to let bad guys get loose just to save Iris.