I've always wondered about the whole Rocco/Roth scene.I think the postings I've read here tend to be pretty much on the money as far as Rocco's reason for being picked,his acceptance of his fate,and the utter impossibility of his escape after killing Roth.
The only thing that I would add is my opinion that Michael deliberately chose a known Corleone associate to do the hit. He wanted it known that he was behind it, and that no one is beyond the reach of the Corleone's. I think he felt that proving Rocco was acting on his orders would be impossible(not just difficult).
Finally, I think that Michael knew the respect and fear that he would gain when an underling willingly sacrificed his own life to do the bidding of his Godfather.