Thanks for all the replies and the insight to the unanswered question.
M.M. Floors, I like your rational about Fredo figuring out what went wrong and to cover his ass he plugged the assassins but think of this:
What happened to Fredo when his father was shot? He couldn't get his gun out and when he did he fumbled it to the ground. Granted the situation but Fredo was no buttman. Secondly Fredo's wife freaked out when she heard the shots and Fredo is seen running after her to calm her down and get her back in the house.
I am stumped.