why couldn't he just put down his gun and become a farmer or something?
DID I EVER TELL YOU HOW FUCKING AMAZING WHEAT IS?
He was traumatized.
The Boss, his everything, got used and betrayed, becasue she put down her gun, so he would never allow himself to do it.
And that's it, he was just and edgy man in denial, until Solid fixed all his mess.
He knew that nobody in the world could beat him in combat. So when you're the best at something, you need to keep doing it to stay the best. He did it for so long that his own genetic material ended up killing him.