Why can't it be an art and a crime?
Certainly tagging is just pure vandalism stemming from a desire to mark your territory, to put your name on the map.
But actual street art? Still vandalism imho. But vandalism can have artistic and aesthetic value!

The very first line of the moderately famous German song Kling Klang by Keimzeit says the following:

Graffitis machen graue Wände lebend'ger, ich wünschte ich könnt das auch.

Translated: Graffiti livens up grey walls, I wish I could do the same.

I couldn't imagine cities without their street art! They would just be huge blocks of concrete.