Why would my pit bull attack and kill our kitten?

We have had our 1 1/2 year old pitt-lab since he was about 6 months old. He has always been around cats, and he's always gotten along with them. In the past week or so, he has shown some signs of aggression (mainly a bit of growling here and there with people he doesn't know), but I was just taking pictures of the dog snuggling and sleeping with this kitten no more than 2 or 3 days ago. I took him to the vet yesterday; they said he was pretty healthy and he seemed fine when he got home yesterday afternoon. All night while I was working, he was fine; my boyfriend and I went home to check on him and let him outside and everything was totally fine. We went out for a little bit, but my dad (who lives upstairs) called to tell me the dog was crying, so he went down stairs to let him outside and give him a snack, still everything was normal, both of our cats were running around the house like normal. We got home about 20-30 minutes later, and found our little 2 month old kitten with her head torn open and her neck broken, blood all over the floor and literally sprayed across the walls. This, to me, looked like a malicious, vicious attack; pit bulls have very strong jaws and my best guess is that he bit down on her head, crushing part of her skull and started shaking her. We are heart broken and so confused as to why he would do this.. it's so out of character for him. He always let her climb on him when he was laying down and snuggle with him and he would just give her kisses. At this point, I am afraid for the other cats in the house and any people that he might decide he just doesn't like. We have a lot of kids in the neighborhood who have tried to come into our yard in the past to "see and pet the puppy" and we are concerned for their well-being as well. We know now what he is capable of, and that (especially) this breed of dog certainly CAN turn on other animals and humans, as he's now growling and snapping at us. I don't know what to do with him, but I know I can't keep him here anymore, he's becoming a danger to the animals and people around him. My real question here, though, is why would he do this? He's always loved all of our cats- they play, sleep and snuggle together all the time- until last night. Just, why? She was such a sweet little kitten, WHY?