It's 'my oppinion' that Michael suspected Rocco's involvement and therefore tested his loyalty by sending him on that ridiculous mission.
So if Rocco were guilty, but he were successful in killing Roth and surviving himself, you think Michael would have let him live because he passed the "loyalty" test?
Suspected is something else than being guilty...
I've always thought Rocco was acting a little suspicious straight after the failed murder attempt on Michael at Lake Tahoe. He gave the impression that he really wanted to kill the assassins, instead of capturing and questioning them, and that's exactly what happened.
My theory is that Rocco could have been aware of what was going on and that he may have even been involved to some degree. I think he was an opportunist. He may have also been approached by Johnny Ola (probably even behind Fredo's back) and would have been promised Michael's organization in Nevada if he wouldn't interfere. Maybe it were even Rocco's own men who had opened the drapes.
I think he was the type of guy who would wait and see what happens and support the winners, no matter what the outcome would be.
And when it turned out that the assassination had failed, Rocco, feeling jeopardized, had to react quickly by killing the assassins before they could fall into the wrongs hands and reveal his involvement.
Michael, suspicious and calculating as always, probably considered such a scenario.
Rocco would be an obvious choice to be involved in the plot, far more obvious than Fredo. He was the one in charge of security at Lake Tahoe. If I were Hyman Roth, I would definitely consider him to be part of the plot. His role would be crucial for the plot to succeed. He would have been far more important than Fredo.
I'm not saying I'm totally convinced that this is the case. It's just a theory I have. And frankly, it's not that unplausible.
