Michael kills Sol and McCluskey because he and Sonny want him dead, and because Michael thinks that the "key" for Sol is to "kill Pop." My question is "How?" Strike one was that the hit men did not kill him when he was at his most vulnerable. Strike two was when McCluskey called off his guards at the hospital but was thwarted by Michael and Hagen's legal maneuvering. After that Vito returned to his Mall, and he was too sick to be out in public all of the time. In fact the only times ever was seen to leave the mall again was when he went to Bonasera's funeral home and when he went to make the deal to sell drugs and bring Michael home. Otherwise he would have been under very tight security, and other than getting someone inside the family to do it a' la the drapes in II, how would Sol have killed him?
As Sol himself said, " I missed my chance."