Oh my God. That's an awful lot of guilt to shoulder yourself with. Please don't blame yourself for his suicide. You didn't know he would go through with it!!!
I know words won't help when you feel this much despair, so let me just give you a hug via cyberspace.

Give yourself a break and try to forgive yourself for this guilt. You cannot take responsibility for his actions. I bet you really didn't know how to take his threat of killing himself and didn't really think he'd go through with it. So don't, PLEASE don't take responsibility for something of which you had no control over.

I'm very sorry about your grandfather. I really feel for you and will keep you in my prayers. Have you gone to a doctor to help you with these situations, or had therapy? I think you need to see someone pretty quick about these feelings. Get a friend to help you. My friend took me to the doctor yesterday when I could barely move due to my depression, but it's what I needed to take that step to get better. Please talk to someone, even call a hotline if you have to.
Let me know how you are doing, and remember that you didn't kill your friend. He killed himself. May peace be with you and your heart.