Excellent point. I’d add that a zombie is both dead and undead — lived, died, and has been reanimated, and in that reanimation become both unclean (because s/he now belongs to two conflicting categories) and relieved of whatever earlier humanity. Because a zombie exists in this awkward, unclean state, and is by definition inhuman, it can be destroyed without compunction and for the greater good.
You don’t go to jail for killing a zombie. Though you do, and should, for murdering a husband, wife, and children, as these two will hopefully learn,