Why would a 1year old dog kill a 9week old puppy?

My friend babysat my male Pitbull boxer puppy, he has a 1year old male pit and they had gotten along just fine, till today. The pit killed my puppy today. I'm almost sure my puppy bled to death or had internal bleeding, he was just a baby, I don't get why they got along just fine, and he attacked my baby... show more My friend babysat my male Pitbull boxer puppy, he has a 1year old male pit and they had gotten along just fine, till today. The pit killed my puppy today. I'm almost sure my puppy bled to death or had internal bleeding, he was just a baby, I don't get why they got along just fine, and he attacked my baby till he killed him. P.S. Idk if this may be a reason, but the older Pit is having puppies with a female dog and she was there today. Any answers will help I'm not mad at the dog because I know they are animals and they simply have different morals than humans. Please anyone help me, I just want to know.
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