If you are so angry, it means that even when he cussed you, your relationship never turned sour. Love is not only uphill and for what I read about him, being furious in a moment was not even worth considering compared to your friendship.

Maybe having this feelings inside your head like a repeat can be harming you or simply creating a never ending circle. Why don't you try writing it with pen and paper? And put in every LINE and with every MOVEMENT all the ****ing hate, anger, resentment against the unfairness of life into it. And then when you are finished, burn it and imagine is all you anger that fuels the flame. And if you need it, do it again about the same or another feelings you have about it.

And you are right, many times life is unfair. It takes what we love the most when it leaves terrible people in here with us, but remember: his son is still here. You can be there for his son, to take his mantle and be the figure he can count with. With him your friend still lives and he would have wanted for you to be with what he loves the most.