I love this line of Mask's bio:
The wearer of the Mask is, to all intents and purposes, invulnerable. Injuries of any sort do no permanent damage, no matter how severe. Although they do bleed normally, the wearer feels no pain (the movie translated this into making the wearer a living cartoon character). They can also produce any weapon they desire out of thin air (again, in the movie this became any prop, a la Bugs Bunny). These are as lethal to normal people as real weapons would be. Since the wearer of the Mask has their inhibitions and conscience removed, treating others as if they were invulnerable as he is, this tends to mean that the Mask kills those he encounters.