I do not know what they were thinking, but if they were thinking that this dog will grow up to be dangerous, and that perhaps someones life or face will be saved by killing it, I cannot fault them for doing it.
Pit Bulls kill more people than any other dog. They maul more people than any other dog. They can be perfectly fine for years, and then some day something will just set them off and they go into attack mode... against their owners, against children, against anyone around.
I've heard theories to explain it, but I have no idea if any of them are true. What I do know is that a lot of people have been hurt and killed by sudden attack mode in these dogs.
I had a friend with a pit bull way back in the early 70s before the average person ever heard of them. I spend hundreds of hours around the dog who was friendly and affectionate. I also saw it turn in a second, tear free from someone’s grip on his leash and maim a adolescent German Shepard who was strolling by. Fortunately it wasn’t a kid, but it could have been. Like I said above, most pit bulls are friendly until one day they aren’t.