It is one of the enduring mysteries of the Trilogy: who murdered the assassins? I agree with those above who state that Fredo did not have, physically or mentally, what it takes to murder two assassins.

Also, it wasn't Rocco; that makes no sense. Rocco surely recognized that his was quite a rising star in the Corleone family. Why would he align himself with several upstart Mafiosi in the Bronx and an ailing non-Mafioso who commanded no troops?

"Generosity. That was my first mistake."
"Experience must be our only guide; reason may mislead us."
"Instagram is Twitter for people who can't read."