Well said, SB!

The novel has a passage that speaks directly to your point about the danger of forgiving treason:
After the Great Massacre of 1955, Kay bolts, and takes the kids to New Hampshire. Hagen goes up there to convince her to return. She says she knows that Michael was behind all the killings, and asks why he couldn't have forgiven Carlo. I'm sorry I don't have the novel in front of me, but Hagen basically says that if Michael forgave Carlo or any of his enemies, they'd be a danger to him and his family all of their lives.
Puzo also made the point that gratitude is one of the shortest-lived of human feelings. He said it in the context of Bonasera getting that call from Hagen to meet the Don at his funeral parlor to attend to the dead Sonny. But it might well have applied to Fredo--if Michael had really given Fredo a pass. How long would Fredo have been grateful to Michael for having spared him--vs. stewing all over again about being passed over, Mickey Mouse nightclubs, etc.?