I do understand your anger towards vandalism, but professional artists will only paint property with the owner's permission. The same way a professional fighter would only fight someone in the ring, and not go around beating the snot out of random people in public.
I would like to hear you opinion of Seed Bombing, which is the planting flowers on property that is not yours by throwing a container of seeds there? Do you consider that "vandalism" or "defacing"? http://www.guerrillagardening.org/ggseedbombs.html