Curlew sir, here is the thing... Your Fraternity brother, obviously had some serious issues in his life. And without knowing anything about him, I can only assume that he didn't know how to ask for help. That isn't your fault in any way, shape or form. But that's what is screwing you up..... You keep thinking "what could I have done to stop this from happening?" And the answer is nothing. Unless you ever had a conversation with him about suicide, you would never have known something was wrong.
Now that is in the past. It's over and done with and nothing you can do will bring him back. But I'll tell you this...
If he loved you, the way you loved him, the last thing in the world he'd want from you now is you ruining your life over his mistake. Almost as if you can here him from heaven yelling at you right now not to worry about it anymore.
Think about it a little, settle your thoughts, and get back in the groove with school!!!