It might be not only that thing. Remember that when Fredo says at first,
"I didn't know it was gonna be a hit Mike, I swear to god I didn't know it was going to be a hit. Johnny Ola bumped into me in Beverly Hills, he said that he wanted to talk ,he said that you and Roth were in on a big deal together... And if they could get a little help and close the deal fast it'd be good for the family."
After that Michael asks only:"You believed that story!"
But Fredo, what an idiot, finds it necessary to draw his attention to one more point. He says:
"He said there was something in it for me, on my own. "
That he, in fact, sold his brother for money, or anyway for his own interests. If he thought he was acting for the good of the family, that might be some excuse in Michael's eyes, but after he said THAT...
And of course all he said after did not help him at all .
Why didn't Vito teach him to keep his mouth shut?