Start right near the beginning, after Vito was shot:

Tom said it would be impossible to gun down a police captain. And, when Michael volunteered to kill Mac and Sol, Tom scoffed. But Michael replied:
"I'm talking about a cop -- that's mixed up in drugs. I'm talking about a dishonest cop -- a crooked cop who got mixed up in the rackets and got what was
coming to him. That's a terrific story. And we have newspaper people on the payroll, don't
we, Tom? And they might like a story like that."

Michael should have followed his own advice because he was right: the newspapers had already played up Vito's shooting. So, instead of Michael killing Mac, he and Tom could have leaked Mac's involvement with Sol the drug dealer and Vito's shooting to the reporters on the Corleones' payroll. They'd have given it a tremendous sendoff. The Police Commissioner, embarrassed, would have had to suspend or transfer Mac, and assign police protection to Vito in hospital to prevent further embarrassment. The entire police force would have been ordered to hunt down Sol and kill him, "attempting to escape," to prevent him from squealing on his relationship with Mac. Michael, his hands clean, could have married Kay and gone back to college. And we wouldn't have had a Godfather Trilogy.