Calo probably volunteered to go on his suicide mission. He was so shaken by the murder of Don Tomassino that he would stop at nothing to take revenge, and gladly gave up his own life. I mean what kind of plan whereby you go up to a man who has an armed body guard in the room, whisper something in his ear and then rip off his glasses and stab him in the neck?
Calo is clearly eager to avenge Don Tomassino at any price.
Unlike Neri and Rocco, Calo is not a businessman and his loyalty is based on something more than money. I think Michael's realization of the difference is largely what spurred his self-reflection at Tomassino coffin.
