This thing reminds me of a scene from the movie Casino, in which the bosses of the Midwest discuss the life and possible threat of one of their associates who has the potential of cooperating with the feds.

One of the bosses defends the guy by saying; "he's a marine and though as hell" etc. But then another boss says: "why take the risk?" ... And that's exactly how the mafia works. Brother or no brother, you don't get a second change.

But the real point is, what did Fredo do? How much was he involved?

If he did pull the curtains or whatever, he should be killed without remorse. Period.

But if he only did the thing we are suppose to believe, like giving some details to Johnny Ola and Roth... He should be killed nonetheless, because that stupidity can be fatal for anyone in the family. It makes him indirect responsible for bad things to happen. And as an underboss and brother to Michael he should have known more secrets about the organisation which could ruin everyone who's involved.