How can operant conditioning help to explain different levels of aggressive behavior between individuals?

Answer 1

There's the attention that they receive from committing an aggressive act (as stated above). Think of all the media coverage and attention from friends and family. Look back at the famous serial killers. Some of them contacted the media and demanded that the newspapers print their letters, to bring attention to their crimes. Some asked reporters to read their letters on the air. Some threatened to kill again if the media did not comply.
Also, look at our prison system. It is a punishment. An environment and action devised to reduce the incidence of a behavior (aggressive acts). People go to jail, not only to keep the public safe, but the criminal system hopes that people will find jail to be so unpleasant that they will not commit another aggressive act.

Answer 2

That's an interesting question, with a link I never thought of before and don't know if I agree with. Nevertheless, here's what I'm thinking.

Individuals are geared toward a certain level of aggressive behavior, and those who are geared toward more aggressive behavior like the attention that they get when they commit aggressive acts. Thus, it's cyclical. They commit aggressive acts, get attention that they like, which increases the likelihood of committing more aggressive acts, and so on and so forth.