Tom didn't have the killer instinct in him. He could delegate the violence, but ultimately, he was a businessman. Look at the events leading up to the climax of GF2 - Tom knew that victory over Roth was already clinched, and questioned why Roth needed to be assassinated. (In the end, he was right - Michael lost one of his best assets while killing Roth.)
Tom was a lawyer, and lawyers always seek practical solutions. They'd rather negotiate than fight. Although it seemed to Tom that there was no reason to kill Pentangeli and Roth at that point, he didn't understand Michael's need to prove to all current and potential enemies that no one gets a pass after betraying or trying to kill him; and that Michael's power to avenge extends even to "impossible" situations, such as being in Federal custody or being met by FBI agents at an airport. As a practical matter: Roth alive under any circumstances was too clever and dangerous to Michael. And, the Government wasn't going to let Frankie "live better than most people on the outside" forever. Though he'd never make a credible witness against Michael, they'd keep pumping him for info until they found something of use against Michael.
As for Rocco: I may be in a minority here but I believe Michael sent Rocco on a one-way mission. He didn't need Rocco anymore. Rocco was Clemenza's man, Neri was Michael's.
