Why does GOD do this?

Well, it works for him so lay the fuck off.

Blaming occurances on "god" is merely a roundabout form of denial and the inability to cope. You can blame things on "fate" if you want, as long as you realize that there is no predetermined plan for existence. He died because his heart failed. He may have been born with a weak heart but it was never diagnosed. Maybe he ate too much junk food. Regardless of the circumstances, that is not divine intervention, it is just unfortunate.

To be brusque, blaming god makes you look weak. You need to become disillusioned and realize that nothing is controlling your life but yourself, your upbringing, your culture, and your society. All of these factors may seem random but can be easily explained by way of your parents. All actions and consequences can be explained. If you continue to pretend that someone with ultimate power did this to your friend, how can you truly cope? Is it fair to your friend?

My point, gruesome, stems from your thread title. God did not kill your friend. A series of unfortunate events killed your friend. Perhaps it all began when one of his father's sperm entered his mother's egg and that particular sperm had a chromosome defect leading up to heart problems. Anything could be at fault, really. It is your job to accept this, and not pretend that he was killed.