In Kill Bill Vol 2, We get to see more of Budd and his story. But it's not the story of a world class assassin, but one of a broken, submissive man who looks like he has nothing to live for.
We see proof of this in the place he lives. A run down trailer home in the middle of the desert. He works at a strip club where his boss treats him like crap. He calls Elle to sell the Hanzo sword.
What happened to Budd that caused him go from a world class assassin with a good lifestyle to what was depicted in his chapter? All the other assassins in the Deadly Viper Assassin Squad were doing fine (until they get killed of course).