I hope I'm misinterpreting what you're saying as it sounds like you're blaming the victims for what ultimately happens to them here. If a bully's behaviour is getting to someone, it's the bully's responsibility and they should be held accountable, end of story.
I wouldn't say its ultimately the bullies fault that the victim is a victim. There are many points in the bully cycle that the victim can break the cycle - even before it begins. But if this victim doesn't stand up for themselves, I'd say they are more responsible for the prolonged suffering they caused themselves. I'd say its 50/50.
Sadly it doesn't happen anywhere near as often as it should, and as others have pointed out, any attempts at retribution are often punished more harshly, which is pretty disgusting.
I've known this too well, that's why I usually prepared a story as to make the bully look worse than he was so I get off lighter, and under one-to-one kind of pressure my eyes water so that makes me look less like the bad guy

I'd also say that it's not natural: it is commonplace, but most people don't do it. I've had trouble with bullies in the past and when I think back, they were all weirdos, people who really didn't have any degree of normal social functioning. Bullying's not so much human nature as sociopaths' nature.
I'd say it is natural, in nature the strong prey on the weak. When it comes to bullying its mostly the exact same just on a personal scale. The bullies I encountered were much like myself but dumb as [censored] in comparison, most only kept up the act when they were in their safety groups - they weren't the worst ones and we formed a friendship (kinda) when one-on-one. Then there was the ones who'd do it regardless if there was an auidence or not - These ones couldn't be dealt by simple ignorance but my showing I'm physically superior, and it turned out these ones all left school earlier, found no employment anywhere or any other form of education and are in fact the scum of society, they were the sociopaths.