Of all four Spiderman cartoons I've seen, I think this is the most adult*.
*If you ignore the silly soap-opera the main characters are always playing. That's not adult.
You've got some swearing, some death, real guns (with real bullets), as well as your standard comic book violence. Mostly the death makes this Spiderman hardcore. In the original animated series, nobody died (except Ben); there were vortexes and close-calls, but nothing terminal. They didn't include Gwen Stacy because they knew they'd have to kill her. Offhand, they've killed Dr. Connor, a few cop/security guards, Electro Boy (at least once, maybe twice), Electro Boy's frat brother, they poisoned Mary Jane in a dream sequence..
Anyway, people die, so this Spiderman is a bit more.. (realistic?) .. than other Spiderman cartoons.