Short form: teenage boys who are diagnosed with "aggressive conduct disorder" shown footage of people hurting other people _like_ it -- it shows in the pleasure parts of the brain when fmri'd.
Next up: Why People Like to Watch Other People Having Sex. Possibly followed by, Mystery of Popularity of Action Movies Solved!
Come on. The _real_ difference between who is a bully and who is not isn't whether you _enjoy_ watching people hurt each other or not. If that was the difference, there'd be a _lot_ more bullies out there. Trust me on this. But not everyone who enjoys a ball game or UFC or WWE or whatever is a bully. Why? Because they don't suffer from impulse control problems. Also, they've channeled who they inflict pain on into socially sanctioned outlets. Like, say, being an elementary school teacher. Or doing reality TV. Corrections officer. I could go on, but why?
ETA: Rereading this, I feel compelled to add: not _all_ people who teach elementary school, work in corrections facilities or do reality TV want to or actually even do inflict pain on other people.