Of course the vehicles in question are phallic and powerful extensions of man, automotive swords cutting a swath through backwoods America . Crime for Larry is an extension of his racing days careening on the edge of disaster, the pumping adrenaline, the connection between quick thought and action - it's, in a word, rush. There's also a moment of liberty - the moment when you get away, not only with the crime, but from the crime scene. Mary, Larry and Deke personify the moment where anti-autoritarian sentiment would end up by 1974, some seven years after the summer of love. Their hard drinking racing days are long gone and they have nowhere to go.
Life is a dirty deed and biting back is their only true recourse.
The decision to rob a grocery store is evident - the trio is pissed off at society and wants a piece of the action without having to actually go out and work for it. Not to say holding a family hostage while committing a crime and then eluding the cops isn't filled with stress. Its a wonder Larry can handle that Charger at all. But the robbery is just a psychological catalyst for what they really want which is the ensuing chase afterward - the thrill. Larry, who's been kicked off the drag racing circuit for reasons never made clear, misses the track.