Basically, I agree. Plain and simple, Fredo just was not smart enough to figure out certain things. He also apparently had a self-esteem problem. Aligning himself with Moe Green illustrates both. He failed to grasp the power of the Corleone family, its place in the underworld hierarchy, probably a version of the Stockholm Syndrome.

"Generosity. That was my first mistake."
"Experience must be our only guide; reason may mislead us."
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