I still remain unconvinced that Rocco's hit was a chance to redeem himself for treachery in Tahoe. As far as the Roth hit being a suicide mission, I think rather than focusing on whether Rocco knew it was going to be a suicide mission, the bigger question is whether Michael regarded it as such. I believe he did, but not as payback for betrayal, but we'll deal with that in a second. Whether Rocco thought it was a suicide mission is irrelevant, IMO. Either he thought he could handle the hit and keep chugging along with the Family or he believes it might take cost him his life, but he's willing to chance that to get his #2 spot back from Neri, as many others have suggested here and I happen to agree with. In either case it's all the same to Michael.

It's also been stated many times that Neri jumped over Rocco since he was Michael's guy, while Rocco was Clemenza's choice. Rocco and Neri are basically the Clemenza and Tessio of Michael's regime. Vito needed both Capos to run the operations of his empire back in the good 'ol days, when everything was illegal. Michael isn't in charge of the old Olive Oil Business, at least not directly, and his new enterprise, Las Vegas gambling is completely legal, hence he doesn't have the same need for manpower as Vito did. They are going to have to partake in some illegal activities to make their casino successful, but one man and his crew are more than enough, and Neri seems to be that man. Plus, while Michael has no real intentions of going legitimate, he is trying to make it appear that way.

If Neri's gonna be Michael's guy, what use does he have for Rocco? He proved to be inept at handling Michael's security on the compound, there's no way a man like him would take the demotion to go back and take over Frankie's operation, especially after he saw how little respect Michael had for the post. The gap between he and Neri will grow even larger as time goes by and that will lead to animosity, which in turn will lead to betrayal. Michael knows this, hence the need to get rid of Rocco. At the time it might seem foolish to send someone as important as Rocco to kill Roth when some other schmuck could may well have done it, but Michael is constantly looking 2-3 steps ahead. At that point in the film, he is barely human, so there is no sentimental reason to spare Rocco. In short, Michael would make a great GM or fantasy sports player. He bought low on Neri and sold high on Rocco, getting that one last great game out of him before he was done.