It is pretty clear that Rocco was always lower on the totem pole than Neri, and I have never bought into the idea that somehow Michael wanted Rocco out of the way, or that Rocco was trying to curry favor in some way to pvertake Neri. If anything Neri was the one who wanted to replace Tom. It wass Tom on whom he did the research Michael used to insult him. Neri was given the job of killing Fredo...arguably the most sensitive of all three, and When Michael sought out Rocco to kill Roth, Rocco was simply being a good soldier and following orders. I always though he botched the job by not having some kind of diversion which may have allowed him to escape. Had he, I am certain michael would have wekcomed him back into the fold, but still he would have remained number 3.