A subtheme of II is how Neri pushes past Rocco, and tries to push past Tom, to be Michael's second in command. Remember: Rocco was Clemenza's protege, Neri was Michael's. By the end of II, Rocco was, essentially, obsolete. So Michael, the master manipulator, set Rocco up in that boathouse scene. After humiliating Tom and stating that "history teaches us that anyone can be killed," he immediately turns to Rocco--not Neri. Now Rocco was on the spot. He answered, "Difficult, not impossible." I'm guessing that he saw his "volunteering" to whack Roth as a kind of hail-Mary pass--a last chance to get back into Michael's good graces, despite the overwhelming certainty that it was a suicide mission.
I still find this a very strange decision, for two simple reasons.
1. Rocco is a caporegime. He's therefore an important member for the organization and it's structure. Other's in the organization may also be angered by that decision and might question Michael as their boss.
2. As he is a caporegime he is closely connected to Michael, who would very quickly be suspected by authorities which turns on heat.
