Stupidity: In the opening robbery, killing the third guard made no sense. Even if they didn't kill anyone, they'd still already committed a serious crime, that would result in lengthy prison time if caught. It makes no sense that they have no problem leaving witnesses when they're guilty of armed robbery, but don't want to when they're guilty of murder. If they didn't kill the third guard, their chances of getting caught would've been the same as if they didn't kill anyone.
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Suggested correction: Leaving an eyewitness for an armored car heist is certainly risky, but leaving an eyewitness for an execution style double homicide is totally different. California had the death penalty at the time this film was set.