I disagree. Sonny, while engaging, was a psychopath. He had a bad temper and he got his hands dirty. He would kill or order deaths on a whim (ie, Bruno Tattaglia), just because he was pissed off. He was knee-deep in the violent side of the family business and it was no shock when he came to a violent end.

On the other hand, Fredo was sweet and stupid and begging for love and attention. In his way, he was a far more sympathetic character.