Eric is being possessed by a deity from another dimension.
The story never outright explains why he descend into arrogant, sociopathic megalomania by the end, but Eric's story about the dream he had at the end implies that, rather than
With Great Power Comes Great Insanity, he was in fact a normal man possessed by a godlike entity from a smaller universe/dimension that wanted to explore another reality. This lines up with several notable moments in the story: the explosion at the apartment complex that leaves only Eric fully unharmed would be the god's "arrival" in the cast's reality, the increasing aptitude he shows with his superhuman powers would be the god slowly recalling how to use them after apparently losing his memory at first (or being suppressed by Eric's) - and his increasingly amoral, detached demeanor (as well as his many acts of violence) are the god working with Eric's memories/emotions and processing them not as a human would, but as someone with
Blue-and-Orange Morality would. Eric remains attracted to Alma and is a little bitter that Hugh "got to her first"? He
rapes Alma and brutally paralyzes Hugh. Eric was quick to jump to violence, but only against bullies and jerks? He starts out "fighting crime", but then decides to just kill everyone that comes at him with any amount of force. Eric always had a soft spot for Sam? Sam is the
only person that Eric spares,
over and over again, even trying to explain his actions to, but in more of a "I guess I'm obligated to" manner than with actual affection. He's just acting out Eric's life, but in a bizarre, exaggerated, and ruthless way.
