People keep trying to make Fredo look "evil" so his death would be justifiable. The truth is Fredo would never go to the FBI against his brother. Even his "treason" was really him being duped by Roth. Fredo thought he was helping his brother and at the same time doing something on his own. Yes he was stupid no doubt but he would never hurt his brother by going to the FBI or having him killed

If Fredo dropped Michael in it, he may well have had to drop Roth in it too. It's never made clear how far Roth's political influence goes, but he has Questadt, on the senate hearing, in his pocket, so I'd say possibly higher than Michael's. Even if Fredo was in protective custody, I would think Roth may be able to get to him. Michael might not have had to lift a finger.
But I agree with OptimumTaurus, I don't think it would ever have crossed his mind to do so. His whole life had been in the Mob, I don't think he knew any other way.
Ironically, I think if he'd immediately gone to Michael after being contacted by Johnny Ola, and maybe said "i'll play along, see what information I can get from them, but I'm with you", his standing in the family would actually have gone up.

Fredo's entire life, from his childhood, he was seen as weak and underachieving.
The reason Fredo gave for attempting to help Roth in his dealings with Michael was that he wanted something "on his own". As in, something not given to him because of his last name.
If nothing else, Fredo did take to heart his father's way, the Omerta, which implies strong distrust of authority figures and an agreement that you would always try to handle your own grievances.
For Fredo to rat out his brother, countless Family friends, and even his own deceased father (he couldn't possibly rat Michael out without explaining their father's way of life as well) would be too much even for Fredo to do. He counted on Michael's sense of brotherly love and forgiveness for Fredo's shortcomings.
Basically, Fredo knows his own reputation, and having "ratted out his entire blood Family to the FBI because of a spat with his little brother" added to his already poor reputation would have been too much for a man struggling to be respected as Fredo was.
That would have worked and Michael would have forgiven him (he may have been really upset and may have even exiled Fredo from anything having to do with the Family business) had Fredo not made it clear how resentful and upset he was over being "stepped over" by his little brother.
Never hate your enemies. It affects your judgment.
-Michael Corleone

You're right because Fredo did have enough on Michael to have put him out of the picture. beep 'loyalty', if Michael wants his brother dead.
However, it's possible that Fredo would be the persona au grata, simply for ratting out his brother. He could become a target. Or end up isolated, guilt-ridden and paranoid.
When Fredo tells him how he was "mostly kept in the dark", I also took it that he didn't know about the Senate Hearings. By then, he was fearing for his life.
I had to see this movie a few times before I began to appreciate it more.
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