It is a crime, obviously. Things don't magically become crimes depending on the employer of the perpetrator - that's the whole point of "a nation of laws and not of men."

It's legal for you to arrest someone if you see them commit a felony. It's legal for you to arrest someone under a warrant if you are deputized by a Sheriff of the court (almost never happens, but legal). It's not legal for you, whether you are employed at a police agency or not, to vandalize someone's camera.