My view is that Rocco did it for the same reason it "had" to be Michael who did the job on Sollozzo and McCluskey; Rocco was a made man, a capo in the family. They knew he could do the job on Roth, and this was the family's only chance to get him. Suicide mission or not, the family comes before everything and Rocco knew that. He did the job personally because he knew that even if it cost him his life, Roth would die.