Anytime I see a bicycle locked up, I slash the tires just to make cyclists seethe. It makes me laugh knowing they're going to put their helmet elbow and knee pads on, their high visibility safety vest, pull out their bike lock keys, say "Ahhh, another good day for cycling! Let's hit those bike lanes today!" only to begin pedalling and immediately realize their tires are flat. Sometimes if I know they're going to be quick - say they stop to get a drink from a cafe - I will slash them and wait around to see their reaction. If I really want to troll them, I knock the chain off its spokes. It never fails to amuse me.