Rocco did the hit on Roth to further his career. As he said, it would be difficult, not impossible.

This is supported by the fact that Coppola wrote a part for Rocco in Part III in which Rocco was still a member of the Corleone family and had only been wounded in the airport. But the actor playing Rocco, Tom Rosqui, opted out saying Rocco died of his injuries. Rosqui was also dealing with cancer at the time and eventually died from cancer in 1991. That is all explained in a couple Godfather documentaries.