In my opinion Fredo did not have to be killed. He could have put him on the shelf. He was a nobody anyways. "Running mickey mouse clubs and pickin people up at the airport". And if the Corleone family was going legitimate who cares how that would look in that life.

Roth had to be killed. As far as I see it If you're responsible for shooting up my house when my wife and children are there then you're going. Nothing left to be said.

Pentangeli wasn't forced to do anything. He sliced his wrists on his own accord. He chose the life, He chose to flip, He chose to kill himself. He could have taken his chances another way but he chose not to.