Let's say Peter acknowledges some evil forces need to be put down for good. And Peter as Spider-man must protect people he cares about no matter what or he will never live it down when even his loved ones berate him in stopping a irredeemable scum bag who had killed people before hand even when after spidey stopped them.
Will it get to the point spidey like Thor or Carol Danvers for example will have to put down threats too big to spare. I believe spidey killing should be considered and make him more human than this pure paragon human marvels trying to make. Plus it makes him not as crazy as batman. I mean he still let's the joker go and is fine with raising a assainan boy wonder, Batman is nuts.
Cap has killed too and he's regarded as Marvel's patron big good along with spidey.