Why Would My Brother Kill Himself Over a Woman?

I just buried my older brother last week and I am an empty shell. 

He was the best man I knew, and he left behind three kids who adored him. All for a low down cheating wife.  

Have any of you ever known anyone who committed suicide as a result of being betrayed by their partner, or were thinking of ending it after being betrayed and dragged through the mud by the one person who was supposed to love and cherish them more than any other? 

Looking back, what were the signs they were considering ending their life? I want to know why I failed my brother and why I did not see what was happening and what he was about to do. Please help me understand this.