Fredo had two primary problems: huge insecurity and frustration.

His behavior regarding Michael derives from those two. Yes, he may have resented his brother, but I don't think it extended to hatred or a desire to see him dead. I also don't think that Fredo was a strategic thinker. His vengeance (if any) was temporal; here and now.

It's all rather complicated when you start speculating about motivations. However, I am surprised that someone who had Michael's intelligence and intuition did not pick up on Fredo's feelings.

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