Along with Edward G. Robinson, Bogart and Bacall, and Lionel Barrymore, the actors playing the less prominent roles do fine work. It’s an excellent movie throughout.
The opening impression is how squalid things seem – the bus, the hotel lobby, the guys and doll at the bar. Nothing is particularly dirty or unkempt, so it’s not clear how exactly the director sets that tone, but it is there. The source of the rot turns out to be Edward G. Robinson in the bathtub. He’s a corrupter.
The music reminds me of Wagner, and good and evil are prominent. Key Largo has a message: Men like Johnny Rocco are evil, and a society that accommodates them is ill; we didn’t fight the war to go back to 1930. Bogart kills the gangsters, deliberately and non-metaphorically. A rationale could be constructed to cover it, but it’s clear what happens. They’re bad guys and Boggie shoots them dead, with a clear conscience and good for him. No one feels the need for any Greedo-shot-first nonsense.