He certainly had it in him to hide his connection with Ola and Roth, and he clearly knew what he was doing would be viewed by Michael as treachery. Even an idiot like Fredo when put between a rock and a hard spot, might have it in him to kill the assassins. Maybe it might have even been an act of revenge; "I thought you guys were just gonna scare Mike, but instead you tried to take him out!"

Roth had clearly compromised Fredo, and sufficiently that Fredo was even able to tell Michael later of who in the Congressional committee was on Roth's payroll. Roth and Ola had clearly let Fredo in on some of their secrets, perhaps to more closely bind him to them. That being the case, Fredo would probably have done anything, including killing the assassins, to prevent the extent of his treachery from being discovered.