Fredo unquestionably committed an act of betrayal.

Had his motivation been jealousy and a desire to supplant Michael, he had to go.

Had his motivation been to help the family because he was too stupid to realize he was committing an act of betrayal, he still had to go.

So even though he would not have been jealous of Sonny as the head of the family, Sonny may still have viewed his stupidity as a danger. Not simply eliminating Fredo pre-emptively because of his stupidity, though. Fredo wiuld have to have actually done something dangerous that was caused by his stupidity.

(I think I liked you better when you were Diane Keaton, BTW)