About the ending

I never thought Stormare's charachter (Grimsrud)was the planning type.

The weirdness of him looking in the medicine cabinet for a cut on his hand during a crucial time in the kidnapping shows the viewer that something is a little off with him.

The killing of Buscemi's character is kind of the same way, he annoyed him and Grimsrud fixed the problem.

It's hard to find any reasoning behind Stormare's character, but Buscemi was stupid for insisting on the car money. Considering that the real money was waiting for him, half of the car's money was not worth the risk given the psychotic and unpredictable nature of his partner in crime.

Although it's hard to determine fully, given the two parted ways with Stormare asking for the car to be split and Buscemi not agreeing to this and insulting him for good measure, it's best to assume it was this that triggered Stormare into killing him. It looks like the wife was also killed after not falling in line with what Stormare wanted (according to him). You can theorise that he was intending this all along or he was toying with Buscemi, but like what has already been said, he doesn't seem the type to make thought out plans. He simply seems a bit of a nut who's the type to adhere to the very basics to get a job done, who can't be trusted to make rational decisions for anything other than that.

I think Gaere was intentionally making Carl angry so he could kill him and keep all the money for himself. He knew insisting on him paying for half the car would make Carl angry and that he's probably storm out after hearing that. So he then took him out when he wasn't expecting it. That makes more sense than him wanting part ownership of a car when $80,000 was way more than what it would cost to buy another car.

I think once the dude said pay for half, Buscemi really should have just taken his damn truck. The car was hot and had blood spatters all over it. He already knew how impulsive Stormare was. The criminals were all stupid in this movie. 10/10