Lee, mahalo for a very thoughtful and timely discussion: "Why did these young men do such violent, cruel things?" I don't know if we'll ever have a definitive answer, but I'll have a go anyway.
I think most young men are born with a killer instinct. (I'm sure many young women, too, but that's a topic for another discussion.) As boys and young men, we hunted for birds and went spearfishing. But for the vast majority of us, the thrill was gone after the first kill. It was replaced by the excitement of competition -- athletic, academic, artistic, economic, political, etc. The killer instinct is rewired into socially acceptable forms. 
My guess is that a common thread running through most of the young men who behave violently toward animals is the inability to compete successfully in acceptable ways. We call them "losers" for good reason. In this case, though, society is the ultimate loser. The wake-up call for parents & educators is to ensure that all young people are winners in some way.