I think that with those words, Michael was continuing his long-lasting insult of Tom. Michael said it in the same way as your boss might tell you if you raise an objection or ask a question "if you don't like working here you can go somewhere else." he would have said that to Tom no matter what action Mike proposed on any subject. They just happened to be discussing murder at the time. That's one reason and a major one that prompted Tom to tell Frankie who compared the Corleones to the Roman Empire "it once was."