Sonny just seemed to have more emotion than many of us, so that emotion impaired his intelligent action.
The Godfather Notebook, containing Coppola's annotations to the novel, says on the front cover* that the three boys are supposed be defined by passion, a "sweet nature," and intelligence, respectively. I think a lot of us have long thought something along those lines. I suspect the novel's Fredo represented the physical man, and they swapped that out for "sweetness" in the movie because of the awkwardness of portraying someone who's characterized by the absence of an inner life.
*All I've read is the front cover.
