All right, here is my theory...


We know from earlier scripts and Fredo's "I didn't know it was gonna be a hit.." line that Fredo was mislead in some way by Roth. The earlier script indicate that Roth/Ola said they were going to kidnap Michael.

We also know that the gunmen were killed by having their throats cut.

What if....

Fredo is everything MIchael said about him but if he believes they are going to kidnap his brother he is going to insist on protecting Michael in some way.

Fredo goes out and recruits some men on his own, insisting they meet outside his window to participate in the kidnapping. Ola's men have to kill them WITHOUT making a sound, so they cut their throats, ditch them and then proceed with the shooting. The shooters then escape from the island on their boat as planned.