Michael would have regarded it as a betrayal--and Tom would have been in mortal danger, and Tom knew it. The tipoff, IMO, is Tom's reply: He says, "Why do you hurt me, Michael-- I've always been loyal to you" [emphasis added]. Notice that he didn't say, "That was a hell of a hurtful thing to say to your brother. Not so long ago, after the Tahoe shooting, you said you loved and admired me as your brother." Tom acknowledged in his reply, that his relationship with Michael was business--and that loyalty, not brotherhood, was both the danger and the issue.