Something I don't get...

it is not explained very well in this film (see The Killers 1946 for a better explanation and all around better movie [in my opinion]).


**SPOILERS**
my explanation is heavily taken from the 1946 The Killers. while the story lines are very different i found that i need to recall the 1946 story while watching this movie because the reasoning and motives for several of the characters appear unrecognizable.

so anyway, the idea is that Jack wants to rob the mail truck but doesn't want to share the loot with the other thieves after the job is finished. because of this, he has Sheila double crosses Johnny by pretending to double cross Jack.

this meaning, Johnny steals the money during the heist with the intention of running away with Sheila. when in reality, Sheila has no intention of this and when Johnny returns with the money at the cabin, Jack intends to kill him and take the money.

in doing it this way, Jack gets to keep all the money to himself while simultaneously blaming Johnny for the money's disappearance.

but the problem is, Johnny gets away so Jack sends some people to take care of him. Jack needs to kill Johnny to make sure that no one else finds him and learns the truth about what happened after the heist.

The reason is that Johnny North is fundamentally honest. He would willingly double-cross Jack Browning because both men are competing for the same woman, but he has no reason to double-cross the others and run off with their money. So Sheila tells him that all the others are planning to double-cross him to motivate him to cheat all of them.

Quite simply, Sheila was playing both ends against the middle. If Johnny had somehow decided to split with her and the cash, her bases would have been covered, just as they were when she took him directly into the lion's den (shot at the Bates Motel set on the Universal lot, btw). Classic film noir femme fatale.