Unfortunately, there is no way to avoid upsetting him or making him angry if you've decided to leave him. But you cannot stay with him just to be kind. Tell him as soon as you are both alone together and in a place where he can let out his feelings without anyone else seeing. Also, do not upset him further by listing all the reasons why you want to leave him. Give him time. As the saying goes, “time is a great healer”. Breaking up is never easy, but people evolve differently and need to lead their own lives.