Butch's death was more or less related to Leonard. He didn't personally make the killing move because he already discerned someone else was more eager to do the killing on his behalf.
However, to ensure his role remained safe and sound, he needed to play his part well. After all, there was still Jeffrey besides him, and possibly more spies lurking around the vicinity.
"I'm not going to pay you for this kill, Relle," Leonard stated. "I wasn't in any danger, and this drunkard is of no threat to me either."
Indeed, the killer was none other than Mirelle, who had long raised her killing desire upon listening to Butch's delusional claims.
She simply hadn't made a move until Leonard subtly gave his permission from the statement he said. The valuable thing that Leonard meant was an end to Butch's miserable life. His death would be valuable because he no longer needed to be concerned about winning big anymore.
