Humans evolved where the strongest survived. You were responsible for taking care of your own safety, and proving you were worth a damn by showing you had some physical prowess to help a tribe or clan. But these days, we're not hunting animals or protecting ourselves from other tribes. We're taught that "violence is bad", that "police" or other security-by-proxy types should handle our safety (even though they're not around most of the time, and they're mostly there to prevent liability to property owners, not protect you from harm). We've also moved to an economic system where money, not strength, is power.
But, in general, humans haven't evolved that much, biologically. We're still the same primitives we used to be, trying to solve all our problems with whatever "hammer" we have on hand. (the old saying is "when you own a hammer, all your problems look like nails". The "hammer" is whatever you are good at..jocks being physically inclined, nerds mentally inclined, etc).
Humans are instinctively inclined to cull the flock if it grows too big. We live in cramped cities, and by natural design we're programmed to try to make others prove their worth. When we don't have any greater threat to join forces against (which is why folks fantasize about alien invasions or zombie apocolypses), we turn upon ourselves...culling ourselves of the "weak".
But, there's a difference between bullying someone, and defending yourself. But teachers and others view it all as "violence", and shun it, so the person defending themselves usually gets into as much trouble as the trouble-making bully that's trying to make others "prove themselves to the tribe". So kids are given negative reinforcement. They are punished for protecting themsevles, for standing up for themselves, etc. So they get pussified.
Likewise, they're given mixed signals by society in general. Assholes are rewarded. The guy who cuts you off in traffic or steals your parking spot or yells at others ... they usually get what they want. Sure, they're a dick, but they're rewarded for being a dick. Most people on top of the world...are assholes. They got there by being cut-throat assholes. Bill Gates is not a nice guy. He can act like one, but if you ask anyone that's worked closely with him, they will tell you he's an asshole. So is Steve Jobs. Warren Buffet may be one, too, although he seems to be up on top due to smarts.
But another thing these folks have in common...they're not pinnacles of physical prowess. They played the economic game, which is still a convoluted, confused mess for most of us to understand, and came out on top. They were rewarded for being assholes, bullies, and now they have tons of money to buy whatever they want.
I guess maybe I'm rambling now, but the point I'm trying to make is that in a way, bullying was a useful function in the past b/c it forced members of a tribe to prove their worth or gtfo. But we're transitioning into a species where pacifism, cooperation and intellect is trying to be rewarded. But since we still see assholes getting to the top, it sends mixed signals. We have a long way to go.