"I can picture in my mind a world without war, a world without hate. And I can picture us attacking that world, because they'd never expect it."
-Jack Handey
Humans aren't passive. I never said they lacked violence. Only a civilization that developed in an environment without any threats whatsoever, would have absolutely no tendencies towards violence. Humans fought their way to survival in a world with a thousand ways to die.
People are willing to commit violence to secure resources, to prove themselves as viable mates, and to protect themselves and others from harm. Most people also have the ability to think forward, so they can understand that, sure, killing this person to steal their stuff will get them their stuff now, but then from then on they'd be known as that person that killed someone to steal their stuff, and they wouldn't get invited to groups anymore. It would have a long-term net negative effect on their ability to get stuff. So the more natural tendency is to be friends, to acquire friends rather than stuff, because friends can work together to get a lot of stuff.
Like, wolves in a pack can commit violence on each other for personal gain, but they don't tear the whole group apart, and don't generally kill other wolves. They have to balance their need for a higher status in the group, with the health of the group as a whole, because the group keeps the members alive.
And of course ratios matter. A person who spends her lifetime being friendly and kind to everyone who lost it once and slapped someone, has very different characteristics than someone that regularly commits aggressive or violent acts and occasionally does something not entirely selfish. We wouldn't just say, "well, kindness and violence are just both part of their essences". Sure, they both have the capability for both, but those traits are not nearly represented in the same proportions, they're not evenly stuck in there just because they have both.
The question of why it's considered wrong to hurt other people is simple from an evolutionary perspective. It's a self-selecting trait in any group animal. If an orgasm has the trait to actively hurt other members of its species for no good reason, then that species won't last long, especially if the other traits of that orgasm work best in group form. It would select itself out of existence. Humans are group animals and so we have the ethics of a group animal- the desire and ability to form bonds, form hierarchies, work together, the willingness to defend that group, and to try to achieve a good position in that group.
The vast majority of humans don't want to hurt other humans for no good reason. Something needs to provoke them, and then even when provoked, many will try to find a way to avoid violence. Aggressive tendencies generally get channeled through fantasies, sports, fiction, some non-violent or low-violence social displays (like bullying, unfortunately, which is usually minor but a times can be deadly), or socially acceptable (but not necessarily logically ethical) forms of violence like military work or police work, where they end up fighting for resources, or protecting people, depending on the nature of their specific work. They also channel it through some religions, through preaching spiritual harm to outside groups, like death or torture.
Only rarely does random violence occur, and it's usually because someone defined their group as only being their local smaller group, didn't think very long-term, and then harmed someone that they considered an outsider. The result is usually prison.
Wars occur generally for resources, and happen when the group is being defined as the nation.
