Graffiti will always be a crime,of course defacing property is vandalism.if it werent illegal it wouldnt be as good or nearly as fun.
It is considered an artform because it deals with colors and dimensions..but theres no reason in putting a title on it different than it already is...even though it has turned into some what of a fashion show there still are true writers who just paint without all the discussion.
Graff expresses anger,happiness or sometimes just to make your mark,its basically like talking with words without ever opening your mouth.There are obvious downsides to it and satisfactions.
I do it everyday of my life and i wouldnt have it any other way.
Im not sure if any of that would help you out with your report,but everybody has different opinions and theres no avoiding it.