Lilo, let's take another look at this long-standing controversy, this time from the viewpoint of Fredo's motivation:

He told Michael in the boathouse: "He [Johnny Ola] said there'd be something in it for me." Given Fredo's burning resentment of Michael, it's credible that his motivation was revenge. So, he probably didn't press Ola too hard on the benefits to him. Ola may have told him that Michael wouldn't be harmed, that all they wanted to do was kidnap him and talk sense to him. Fredo would have closed his ears to the speciousness of that tale because all he wanted was for Michael to be taken down a few pegs. And, when he said on the phone to Ola, "You guys lied to me," he may have meant that they'd promised there'd be no shooting.

But: suppose what was in it for Fredo was to become the new Don? That viewpoint is supported by his next statements in the boathouse:

Michael: I've always taken care of you, Fredo.
Fredo (voice rising): I'm your older brother, Mike, and I was passed over.
Michael: That's the way Pop wanted it.
Fredo (screaming) It ain't the way I wanted it!

Even Fredo had to know that he'd never get the Donship while Michael lived. So, he had to be an active, knowing, willing participant in the plot to kill Michael at Tahoe. And, "you guys lied to me" may have meant that they'd assured him that Michael would be dead.

If you believe, as I do, that Fredo was a key player in the Tahoe shooting, you also must believe he had to be a part of the Havana treachery:

First reason: blackmail. Ola says on the phone: "Your brother's not gonna know we talked"--a thinly veiled threat. Fredo's coglioni were in their pockets.

Second reason: greed. I'm guessing that, after Michael met with Roth in Miami, Roth called Fredo: "Your brother got lucky at Tahoe. But the important thing is that he thinks Pentangeli was behind it. He doesn't suspect me--or you. And he thinks our deal is still good. Why, he's agreed to come to Havana and bring $2 million as a 'gift for the President.' I'm the most powerful gringo in Cuba. As soon as I get the $2 million, I'll have Batista arrange to have Michael whacked, and make it look like he was killed by the rebels. And then you'll be the Don. All you have to do is go along" [as if Fredo had any choice].

And (breathes deeply), let me go a step farther:

Fredo's active, knowing role in the Tahoe assassination attempt may have been to admit the two gunmen to the estate, lead them to Michael's bedroom window, and help them escape. But, escape would have been problematic, given the size of the estate and the high level of security. The gunmen easily could have been captured, and forced to tell what they knew--implicating Fredo and Roth.

So, I'm guessing, to protect himself and Roth, Fredo killed the Tahoe shooters--dead men tell no tales. Some people on this board, seeing a red mark on the throat of one of the gunmen, conclude that he was stabbed or garrotted. Not even Luca Brasi could have taken on two trained, armed killers with a garrotte or a knife, much less Fredo. But I'm guessing that the red mark was a mistake--and that Fredo shot the two gunmen, the sound of his gunshots masked in the general cacaphony of machine gun rounds.