War and violence are not just human nature, they're the nature of all living things on earth (and probably in the universe), and they are common sense. Organisms complete instinctive with their own species and with other species for important reasons like controlling resources down to almost pointless reasons like fighting over a mate with another male.
With regards to humans, it defies reason and logic to expect that you can have these two things occurring together:
1. No violence, hostility or killing, but only peace and cooperation.
2. Human beings being allowed to exercise free will.
You can have total peace as long as you have total control over all people and can force them not to have conflict with each other.
You can have humans allowed to live naturally, making decisions based upon our right to free will.
But you can't have both.
A lot of people don't want peace, they want power, or simply they want to hurt people or to kill people. They enjoy it, for some people it's all they know, and for many people gaining and holding power is literally all they care about and they would start 100 wars to make sure they keep power.
If you disarm people of all weapons, the most physically powerful people or the people that form groups the best will control the others with hand-to-hand combat alone. Sometimes there are really good reasons to have physical conflicts with people and the ability to access weapons is important, like if you find a pedophile molesting your child or if someone is trying to home invade your house.
In these situations sitting down and having a chat about the problem is not a realistic solution.
You're not the first person to think it would be nice to have universal peace, but it's not going to happen without removing free will completely, which is infinitely worse.