Michael had nothing to lose by killing Roth in such a public fashion.
1) It signified to the other Families that no one,regardless of their power or position, is beyond Michael's vengeance.
2) No way, no how, does Rocco escape alive. No direct testimony from the shooter to implicate Michael.
3) Let's say Rocco does survive. He knows as well as anyone what happens to potential witnesses against Mike. I say he keeps his mouth shut,takes a deal brokered by some (unseen but friendly) Government/Law enforcement contacts,does his (less than life) time and retires to a comfortable existence.
4) Lastly,let's say Rocco is captured alive,flipped by the Feds and decides to testify against Mike.All you have is (at best) the uncorroborated testimony of an accomplice, which is admissible in exactly zero Criminal trials. No one in the room when Mike gave the order is going to talk. Given the whole Cicci/Pentangeli debacle, no prosecutor in his right mind is going to publicly "harass" Michael again,especially when he would be playing with an even weaker hand this time around. Q; "Mr. Corleone, isn't it true that Mr Lampone is a high ranking member of your Crime Family" ? A: No it is not" Q: "Did you order Mr. Lampone to murder Hyman Roth"? A: That is a complete and utter falsehood" "Thank you Mr. Corleone. The Prosecution rests".
