Humans are actually a very violent species. It's programmed into all of us. Most of the time it's quiet, latent and hidden deep down inside. Some people even deny it. But it's there all right.
Wars will always happen. You can't fight millions of years of evolution.
Reducing the profit motive would help.
"War pigs" make money by financing and arming both sides of many conflicts.
It's a really ugly part of the modern world.