My four year old male Pit Bull killed my male Pit Bull puppy I brought into the family. What should I do?

I adopted an adorable Pit Bull puppy when he was seven weeks old. He was soon approaching his fourteenth week of life this week, until my three year old Pit Bull decided to kill him today while I was gone. He is very territorial, but has never shown any signs of being a killer or even behaving aggressively. I arrived home today to find a note on my front door from Animal Welfare saying that my older Pit had killed my Puppy. My neighbors saw everything and called Animal Welfare terrified. My three year old Pit had blood from head to toe and my poor baby was lying there already gone. I really do not feel it was an accident. I really loved the puppy very much and never would have expected such a horrible devastation to occur. I wish I could have had some kind of a sign to protect him ahead of time. I feel responsible for being gone and not being able to save him. I just don't know what to do at this point? I am not sure what my dog is capable of now that I have witnessed him take an innocent life. I have cried all day questioning whether I should keep him, give him away, or put him to sleep. All of these are horrible options, because I want to keep him. I love him so much, but I do not feel that I can get past this. If I ever do get past this, I don't see it being anytime soon. What should I do? Will he attack again or was he jealous of my puppy? Please help me this is a very tough decision for me to make, but I am willing to sacrifice for others safety. Thank you for you help and god bless! This too shall p with time!