Did Michael believe it was a suicide mission? Maybe not. When he decided to kill Solozzo and McCluskey, he was taking the same risk. There could have have been gun-men at the restaurant who could have killed michael. IMO that is the significance of michael's "anyone can be killed" remark. He did what was necessary at that point. So he expects Rocco to do the same now. Ofcourse there would be FBI men sorrounding Roth. But similar risks were taken during Barzini's murder by Neri, the whole shoot-out during the attempt on Pentangeli's life etc.