Overall, I enjoyed the film especially the ending. But I didn't get two things:
Why does Querelle kill the sailor?
Why does Querelle betray his so called best friend / lover and report his location to the police?
So much to see, so little time!
For the second one, because if Gil is accused he's off the hook. Even if Gil was Querelle's best friend, his lover, Querelle is still a selfish brute. Everything he does is only for his benefit.
Life is far too important a thing to ever talk seriously about it - Oscar Wilde
in the world of Genet you don't need a motive to do a terrible thing. As for friendship betrayal it's the common theme in Genet's books. There is a character in Notre Dame des Fleurs who adores the betrayal because it makes him greater, more significant. As for the murder well some people are just natural born killers, i guess. In the book it was told that Querelle just felt the killing in the air. Also, it might be an allusion on suppressed sexuality.
shareThe voiceover relates why Querelle goes on to betray Gil:
"In some obscure way Querelle understood that love is voluntary. You have to want it. When you don't love men, letting yourself get *beep*ed can give you pleasure, but to *beep* men, you have to love them, even if only at the moment you're *beep*ing. So if he was to love Gil, he would have to give up his passivity."
Querelle betrays Gil precisely because he has feelings him. Loving Gil would require him to emerge from his carefully cultivated passivity - something he is not willing to do.