Since I know a lot of real dregs of society I could tell you at least one person’s reasoning, or at least how he explained it to me when we debated it. However I don’t believe you want to know how sick and twisted minds really work. It’s morbidly fascinating, but you know what Nietzsche said about fighting monsters. “if you gaze long into the abyss, the abyss also gazes into you.” Do you really want to know how such people think?
I guess I will just abbreviate it down to this. Sociopaths often lack empathy, and some warning signs are animal cruelty and blatant disregard for the law, among other things. When you see someone doing something like dumping these animals, it is illegal. The person most likely has no empathy, repeatedly breaks the law, and most likely is probably doing much worse things than dumping puppies. Call in their license plates, description, and the incident to the cops immediately. You’ll be doing the world a favor.