Why did he hang himself?

I'm so sorry this has happened to you. You do not have any responsibility for this happening. When someone has that innate depression and doesn't get help this can happen for any reason this just happened to be the time. He was responsible for getting help for himself and he didn't. God forgives I truly believe. He was not a bad man and nobody can be blamed for what happened. Maybe it would help for you to have a session or two of therapy to talk this out with a professional who will be more able to help you. Please take care, my prayers and thoughts are with you.