Mr. Blue and Mr. Brown (Edward Bunker, Quentin Tarantino)
Sometimes people need to die. If everything goes haywire at the heist with "seventeen blue boys" just outside and Mr. Blonde going crazy on the civilians, it wouldn't really make sense for everyone to make it out alive. The job of dying for the sake of plot credibility falls to Mr. Blue and Mr. Brown, who eat it before they really get to say a whole lot.
In fact, rumor has it that Tarantino wanted to play Mr. Pink but then relegated himself to a smaller role when Buscemi applied for the part. It was only later that he wrote the diner scene so that Mr. Blue and Mr. Brown (mostly Mr. Brown) would actually get a few lines (and most importantly get to say "dick" a lot).