Arguably self-defense. If we're looking for active crimes, how about covering up the various crimes of wizards? Not just activities due to Voldemort, although a close read of canon likely would turn up many crimes where the wizarding world decided its own punishment was good enough and never even allowed the harmed muggle's survivors any input into the process, but just bad guys - like the ones who enchant muggle artifacts. Do you think Arthur Weasley ever turned over to muggle police any wizard whose artifact killed or injured a muggle? Not likely.