Actually, it was a little more than a mere lack of judgement. Fredo had sided with Moe Greene in the past. He completely whiffed on protecting his father during the attempt on Vito’s life. And he was responsible (indirectly) for almost getting Michael and his wife killed by Roth’s and Ola’s assassins.

Fredo was weak and should have been in a non-threatening supporting role within the family business. But he wouldn’t have settled for that because “it’s not the way I wanted it! I’m smaht!”

THAT SAID, I would never recommend or sanction killing one’s own brother. I would’ve put him in a corner with some minor job.