I think the exchange, supposedly about Roth, was actually about Fredo, and that Michael was trapping Tom into supporting the murder of their brother (You gonna come along with me in these things I have to do -- or what?).
That's an interesting point, MP.
I believe that Michael didn't tell Tom he was going to whack Fredo, but you can infer that "Are you gonna come along with me in these things I have to do" definitely included Fredo's death, whether or not Tom knew about it.
Michael probably thought that the fewer people who knew about the plan, the better. And he also must have assumed that Tom would have figured it out himself.
But I also think Michael
might have had two other motivations for not telling Tom:
--Bad Michael: He couldn't be absolutely, positively sure that Tom wouldn't warn Fredo, out of brotherly concern;
--Good Michael: He wanted to spare Tom the agony of knowing that Fredo would be whacked.
