if you recall the scene; they were all seated except Michael, who was restless and animated out of agitation, and Tom's ill-timed comment was the little nudge to get him off; he may not have intended to talk so openly, but Tom's defeatist and skeptical approach got to him.
DW:
One of the notions that I've put forward in some threads here is that the
entire boathouse scene is about the impending murder of Fredo. The scene commences with Tom stepping into the boathouse, and turning to look at Fredo
in the background.
You are correct: Michael is extremely agitated. Note that Michael as a rookie gangster lit Enzo's cigarette with perfectly steady hands. But in the boathouse, Michael is constantly fidgeting with a cigarette lighter. Something more than business is in his mind.
The dialogue makes perfect sense in this context. Michael has to kill all of his enemies because he cannot kill his brother and leave other enemies alive. He gets Tom to give him a blank check of loyalty "in these things I have to do." And when he says that history has taught that you can kill anyone, it might be that he is contemplating violating the eternal taboo against fratricide.
To kill your father's son is to kill yourself. And next to that, Roth is small potatoes.
