I don't think anyone, including Fredo, expected Mike to beat the Senate Committee.
Pentangeli's recanting was out of the blue.
I think Fredo, like pretty much everybody, figured that Five Angels would bury Mike,so he had nothing to lose by telling Mike about it.

The one thing I feel strongly about is the utter evil and cold-blooded decision by Mike to kill Fredo.

I agree that Fredo did not deserve to be killed.
Now,if he took a shot at Mike and missed,OK, then all bets are off.

But he didn't,and Mike showed his true colors by killing him

.