Some questions about character and motivation

I've read the original W.R.Burnett novel many times, and seen the film version often, but there are still some aspects of Little Caesar that puzzle me.

All the characters are much more hardbitten and cynical in the book, and the comic tone that tends to predominate in the movie is largely absent. The story reads as a realistic, downbeat crime drama of believable people and a rough, genuine underworld.

What do viewers make of the opening sequence of the gas station robbery? Rico gets out of the car, forces the attendant into the building, turns out the lights, and we hear several gunshots. The next sequence shows Rico and his pal Joe eating in a diner, and setting the wall clock back to estabish an alibi.
Do we assume that Joe was driving, and he approves of Rico murdering the filling station attendant? He doesn't seem too disturbed by anything that's happened. Did Rico actually kill the man, or do we assume he was just firing a few shots to scare the guy into handing over the money? No one ever mentions it in the rest of the movie, and there's no dramatic moment of retribution, like a police detective from the state they were in, arriving in Chicago unexpectedly, to arrest Rico for the murder of the gas station man.

The killing of Crime Commissioner McClure during the night club robbery is also a bit confusing, in that it all happens quickly and for no obvious reason. Is McClure actually reaching for a gun on his hip at the moment Rico shoots him? If not, then the killing of McClure is truly senseless and ultimately self defeating, since it brings disaster on the whole gang.

There's no obvious reason for Joe to have taken up with Rico to begin with, because Joe never seems like much of a criminal type. Once he meets Olga and gets the dancing job, he seems to lose interest in gang activities pretty fast. Olga tells him he's not that kind, and doesn't belong with the bunch of hoods he's been hanging around with. ( for that matter, Tony seems pretty unenthusiastic about taking part in the robbery, also. )

Any ideas? What do other people think about some of these actions and the motives behind them?

And when he crossed the bridge, the phantoms came to meet him