Looking at the dialogue in that scene and listening to it, Michael is not neccesarily making a reference to killing a President. Yes, Tom makes the analogy that killing Roth while in custody would be like killing a President, but Mike is not necessarily making a reference in his reply to the killing of a past President! Maybe by saying to Tom " You Suprise me, if anything in this life is certain -- if history has taught us anything -- it's that you can kill anybody, Mike could have really been saying, as an inside message to Tom, " You suprise me Tom, if the history of The Corleone family has taught us anything, it's that you can kill anybody. You suprise me, we killed a Police Captain, and the heads of the five families when everyone else thought that we were weak and that it was virtually impossible for us to do so!"


MICHAEL :That's where I want it met.


TOM :Mike that's impossible -- they'll turn him over to the Internal Revenue, customs, and half the FBI.


MICHAEL :It's not impossible. Nothing's impossible.


TOM :I'd be like trying to kill the president -- there's no way we can get to him.


MICHAEL :TOM, you know you surprise me -- if anything in this life is certain -- if history has taught us anything -- it's that you can kill anybody.


Don Cardi