Because a hero without empathy or restraint is literally a monster, ranging from dangerous to an extinction-level event depending on who it is.
Spider-man is, at a minimum, an assassin you could never escape who could murder you on a whim. At his worst? He's the creative equivalent of "The Fly pulling the legs off of a person". He could do A LOT of damage before choosing to end you.
See also: the time Peter beat the shit out of The Kingpin, in prison, to prove a point.