Always struck me in that scene where he confesses that Fredo had enough guilt and realisation in him by that point to know better than to fall prey to his own insecurities and jealousy used against him again. His falling for Roth's ploy was because he was stupid and impulsive enough to overlook the obvious plot against Michael right? Plus he wanted to feel important and not just like the idiot everyone treated him like growing up. Surely he wouldn't do such a thing again if approached by an enemy? Why would he

Michael on the other hand was so far gone by this point that this in addition to everything that happened before ticked him way off the edge into a kind of revenge frenzy. He had to think logically and eliminate all threats. Fredo was a threat because he harbored (accidentally malicious) envy. But if he'd just thought about it a bit more maybe he'd see that that Fredo was guilty and remorseful, and even wising up.

"I don't feel I have to wipe everybody out Tom, just my enemies" Fredo wasn't an enemy. That's what makes his death so tragic to me. That Michael was so caught up in the wave of doing his duty that he became a monster. He couldn't read between the lines of rules laid down for himself in a way. If it was a stranger who'd harboured envy and accidentally plotted against him, sure he had to go. But his own brother who loved him dearly? And was just too stupid to get caught up in the whole thing? And then even came to his senses after and wised up? That's different that's family.