"They're on such a power trip about what they're doing that it doesn't dawn on them how disgusting it is," said Stanford's Siegel. "A person can get set to such levels of psychological arousal that ordinary life can seem kind of drab, and the only way to keep yourself feeling kind of good is to do things that are dangerous or anti-social."
Children who are violent toward animals -- killing a stray dog or cat, for example -- have a heightened risk to be cruel to people, said Scott Lilienfeld, a professor of psychology at Emory University.
"It's well established that one of the best predictors of later violence is cruelty to animals," said Lilienfeld, who declined to comment specifically about the Marines in the puppy video because he has never treated them. "For some kids, [harming animals] might give them an immature sense of power; they may get a kick out of seeing the animal suffer."
But in contrast to this latest incident, most kids would rather hide away and torture the animal, careful not to be caught by parents or adults, said Lilienfeld.
"It's going to be the kind of thing where nobody else is around -- kids know it's not socially acceptable," said Lilienfeld.
But even the worst kids don't always hurt animals, he said, which may demonstrate quite how much it takes to harm or kill a dog.
"Cruelty to animals is rare and frankly even a lot of bad kids love animals," said Lilienfeld. "Most people -- even callous kids -- have a strong immunity to really hurting dogs and cats. You've got to be really bad."
This trend also works in reverse, he added, meaning that individuals who harm or kill humans may also be more inclined to then hurt animals.
"One of the horrible problems is that we have men and women coming [home] who have participated in horrendous violence," said Stanford's Spiegel. "They've crossed the line [of violence] once, and it makes it easier to cross the line again."