Thinking about the many films that I've seen that I would consider slapstick, I have to say that the question of cruelty and permanent harm or real hurt are directly connected to one of the tenets of slapstick–the situation must clearly be make believe. For instance, in Laurel and Hardy's Flying Deuces, Hardy dies at the climax and his soul (an angel with wings and playing the harp) ascends to heaven–definitely a permanent harm but tempered by an element of the fantastical.
Additionally, it seems that slapstick is also marked by a struggle for power and dominance. L'arroseur Arrose would not be nearly as funny if the gardener had sprayed the boy. This is evident in the many underdogs who are harassed by bullies in slapstick, who then go on to turn the tables and triumph in the end.