Michael's favour
Why did he lend money to Johnny Boy in the first place?
"You get me the real money, and i'll bring you the real diamonds."
Anyone?
He's so desperate to get the money back that it can't have been to get rid of Johnny.
"Throw out all that psychological mess."
I guess he thought that Johnny Boy would pay him back with interest, and that if he didn't Charlie would take care of it. Despite his ambition, he's not depicted as being hugely savvy. Remember the Jap adapters?
"Reality is the new fiction they say, truth is truer these days, truth is man-made"
As many times as I watched the film, I can never figure why Michael loaned the money, either. He KNEW Johnny's reputation, one assumes that the four main characters had known one another since childhood (although Michael didn't seem to know Teresa). Although the others refer to Johnny as "that kid," I think it's more because of his attitude rather than chronological age.
"We're fighting for this woman's honor, which is more than she ever did."