Yes, fraticide is horrible (and, as Lamberto said, "Those are terrible crimes and it is just(ice) that you suffer." And I'm sure that anger and revenge entered into Michael's decision. But, as a matter of business, Michael was dealing with a brother whose betrayal almost resulted in fraticide of his own. Plus, he could have saved Michael from the perjury rap he was facing. Sure, he could have made arrangements to see to it that Fredo never was in a position to learn anything of use to a strong foe. But, he'd still have the memory that his dumb, incompetent brother nearly had him killed. Not a big incentive to let him live.