I read a really good little bit of a ssection on "Spidey Kicks Butt" that explained why Spidey, or even batman,, would never ever kill...
Something like:
"But why doesnt Peter just kill Notrman and just end it, or batman with the Joker? Because spidey knows that if he kills 1 person he'll kill again and again until he actually is the menace that JJ thinks he is!"