saitama is kind of selfish, doesn't care much about his own reputation (a good one is better though, for selfish reasons), is a devote follower of mammon.
wouldn't kill humans without very good reason, probably. kills monsters without much remorse, but waits for them to commit a crime first.
he's also really lazy in odd ways. trained himself near death, but can't be bothered to save the city as long as his own house is saved.
cares a lot for his friends, but not enough to care about their belongings. has no qualms about using his friends for his own gain whenever possible. as long as it doesn't ruin the friendship.
he isn't a good person at all, but still so wonderfully normal. except for the ridiculous power.
and totally too lazy to go all out evil.
if he went evil, he'd just let his enemies kill each other, much easier than running around all day eradicating folks.
he'd probably consider evil though, if it didn't hurt common folks too actively, and gained some semi-legal money.
don't you think?