With great power comes great responsibility. That's the motto by which Peter Parker goes about his Friendly Neighbourhood Spider-Business, and it's connected with a bunch of other superheroic provisos: he has the secret identity in place to keep his loved ones safe, all that guff, but tantamount amongst those guidelines? He doesn't kill. It's not really in the fun-loving family guy's nature.
It's possible that's partly to do with the origin story, with Batman's own influential childhood tragedy leading him to take a strict no killing (and no guns) rule. Spidey's never really ever come out and said as much, because he never really had to: it's kind of assumed that a superhero in the Marvel universe who isn't The Punisher doesn't kill people in cold blood. Except, well, it's even easier to play fast and loose when you're working with pretty vague rules anyway.
The Dark Knight has managed to break away from both of his central tenants on multiple occasions, so what's to stop the web head? Besides making the snappy one-liners a little more gauche, anyway.
From misadventures to accidental deflections to straight up erasing a guy from existence - here are ten times the nominally amazing Spider-Man was forced to kill.