As D states, Fredo was dumb and wein. So, the remedy was to exclude Fredo from family business which Michael finally did. But fratricide is quite a leap for anyone to make and was unnecessary. Vito couldn't even bring himself to stop Carlo from abusing his daughter; he couldn't bring himself to murder Carlo because of Sonny's death. He definitely could never engage in filicide.
Especially when considering that family meant EVERYTHING to Vito, since he had his father, mother and brother all stolen from him.
