The family's urgent objective was to prevent another attempt on Vito's life. They needed to eliminate Mac and Sol ASAP. Suing McCluskey for assault would have been, at best, a long, protracted process. Vito would have been dead long before any suit came to trial. And it probably wouldn't have because all the witnesses were cops, who'd put up the famous "blue wall of silence" to protect Mac.

A more credible alternative to Michael killing Mac and Sol would have been for Tom to use the family's newspaper contacts to place a story about how Mac took bribes from organized crime and was in cahoots with Sol to kill Vito--attest the removal of the Corleone troops from the hospital. Given Vito's prominence, the police commissioner would have no choice but to assign uniformed cops to guard Vito, and to suspend Mac pending "further investigations." The Corleones then could have used their considerable political clout to have one or more of their judges and politicians call for Mac's dismissal from the force--and to round up Sol and deport him. But then we wouldn't have had a GF Trilogy.