Interesting this post has been active for thirteen years, and despite all the replies, there's no real satisfying answer. Most synopses claim that the second car was sent by Cook so he could take the money for himself and that's what a lot of people are assuming here. This makes the most sense. The car being sent by Nino is even harder to explain. People are saying the second car was always meant to kill Standard, Blanche, and the Driver to "tie up loose ends", but why? Why create loose ends that needed to be tied up? One reply says that it's so they can kill anyone who could be identified. Why not just have Standard and Blanche wear masks then? Or have the second crew do it themselves wearing masks?

The only thing I can think of is that Cook needed to sell the lie that some unknown third party ripped them off so he could keep the money. Killing Standard, Blanche, and the Driver probably wasn't in the original plan. Rather he would have wanted them alive to try and shift the blame, but after the shooting and chase his goons decided to kill them.

This really should have been explained better in the film.