[What did he actually gain by murdering Fredo? People being more terrified of him?
Michael is a thoroughly reprehensible character. In his mind, if he showed any mercy or forgiveness to anyone who "injured " him--even his own brother-- (as he put it to Cardinal Lamberto in III), it would be viewed as a sign of "weakness" by enemies and friends alike--and would endanger his life. "This was the business he chose."
Again there really is zero evidence that shows Fredo wanted to kill him
Yes, If Ola had flat-out asked Fredo to help in the murder of his brother, Fredo would have said no. So, Ola played him--Michael was being tough on negotiations, they needed information, bla-bla-bla. But, Fredo just closed his eyes,,ears and mind to what was going to happen when Ola told him to open the drapes--especially since he was promised "there'd be something in it for me."
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