Since plaw has referred to my earlier theory on this question:

A subtheme in GFII is how Neri attempts to push Hagen and Rocco aside to become #2 to Michael. Many examples, but the best one is in the penultimate boat house scene. We see Neri lead the discussion, slouched in a chair, talking about Roth's moves, giving a smug little grin when Michael confronts Tom with his knowledge of another job offer and Tom's mistress (Neri probably was the source of that knowledge). Rocco just stands silently, like the mere bodyguard he was in most of GFII. After Tom objects to Michael's plan to have Roth killed, Michael further humiliates him: "You know, Tom, you surprise me...if there's one certainty...it's that you can kill anyone." The he immediately says, Rocco?" Notice that he didn't say, "Al?" Michael put Rocco on the spot: after Tom's humiliation, what could Rocco say, except, "difficult, not impossible."
So, why did he take on this apparent suicide mission? Because Michael, in effect, left him no choice. And, perhaps, because he saw it as a last-ditch attempt to push ahead of Neri. IMO, it was too late: Michael had decided that Rocco was expendible, Neri wasn't.

Geoff, SC or DC: Please wake up plaw--my repeat posts are the greatest soporifics since Quaalude.