I don't really believe in original sin...but I also don't believe that kids fully understand the consequences of their anti-social actions and that this ignorance is ingrained. Even though I agree that young kids can show empathy and so on, I'm not sure that it's part of their brain and moral development to really be able to step into other people's shoes for a long time.
I see aggressive behaviour or power-play behaviour a bit like I see a baby dropping food off a tray. It's experimenting with the forces around us - in the baby's case, gravity. In the kids' case, social capital (withholding, and giving), tribal pecking order, etc. I think some kids may experiment once (or not at all), others are going to experiment over and over.
What shapes them is how those around them respond to that experimentation. Fighting on a school bus where the bus driver can't notice makes it clear that there will be no consequences. So, over time, it escalates and even becomes part of the 'riding the bus' culture. I don't think we can blame any one environment...and actually I've worked with kids who were very kind in one environment, and not in another. Bullying could be reinforced at home, in the media, at school, on sports teams, etc. Or at least not negated.
In my experience not all the kids who are bullies grow up to be bullies. Some do, but a lot meet up with some kind of insight or pressure. There are definitely adult bullies, but some of them weren't bullies as kids.
When I was in school in the 70s, I have to say that bullying was basically acceptable. Not only did teachers participate in it, I bullied on a couple of occasions where we were hauled in and in 2/3 cases that I remember, the attitude of the teacher was more along the lines of "don't do that, even though so-and-so is a total screw up" than "what's wrong with you." However I was mostly on the receiving end of the bullying and a few things that happened to me that didn't result in very significant consequences would probably produce a suspension or expulsion these days. There's tons of bullying in literature going back. So I don't buy that it's getting worse. We may be noticing it more.
I do think some of the new anti-bullying stuff is making some progress, especially programmes that focus on the role of the bystander. It will be the bystanders (the majority, whether child or adult) who end up setting the tone.