Vincent and Butch
Vincent is a professional killer with a code of conduct and honour
He had just been part of executing Brad and was there for Jules killing "flock of seagulls" - he then killed Marvin, albeit by accident, but he showed no remourse
After killing two and witnessing one, as part of his job, he goes for breakfast then to the club to meet Marsellus and deliver the case
He is in a bar owned by his "Firm" where he respected, a feared hitman, and Butch is just a Paluka, being paid to throw a fight, making him a piece of shit, a coward
Also Vince kills men, and Butch has never killed anyone, he is a "virgin" in a hitman's eyes
Butch is no no warrior like the family men who owned the watch before him, he has never been to war - his war is the ring, but still a game, and now he is giving up what honour he had to throw the fight for money
Butch had already arranged and decided not to throw the match before this meeting - his phone call about being the "rich and prosperous" as well as having the girlfriend fetch the stuff and have an escape route planned shows this - he made the bets on himself to win and took the bribe money to make a nice fund for himself
The war watch story is also key - he had the watch and wanted to be a warrior so would never have thrown the match
The reason he is so calm with Marsellus doing the "pride" speech is because it all meant nothing - he was not going to throw the fight
He kills Floyd in the match - he is now a killer, even if this was incidental and accidental
Point is though, once he knows he shows no remorse - a capacity he always had
When he meets Vincent in the apartment he is no longer a "virgin" so he is able to kill Vince with his own gun - now a true Warrior
In the shop he could leave Marsellus to be killed, and that would end the search for him - his honour as a Warrior, which he always had, compels him to go back and rescue the helpless, not innocent, but helpless man, who is being tortured and will then be killed.
He kills one of the men and is ready to kill the second before "step aside, Butch" - he has gone from a "virgin" to a warrior who has killed two enemies
Keep in mind he was also ready to kill Marsellus with his own gun before being stopped