Yes deflating someones tires is illegal.
Even though you actually do not destroy the tire by deflating it with the valve you still are vandalizing someone else's property.
If you get caught deflating someone's tires you likely will just get off with a fine and a court date but in some places you could be arrested and go to jail over deflating someone's tires.
But if you actually slash someone's tires then you likely will get arrested.
A group of teenagers around here slashed a neighbors tires and they all got arrested and had to serve 18 months in Juvenile Detention over it.
They also got a $2,000.000 fine each.