unmercibly

Possibly 'unmercifully' or 'mercilessly' -- that is -- without pity. Said of something or someone who will continue to attack until death, and if it isn't their own death, even <i>that</i> may not stop them.
Personally, I think the Zombieslayer made more of a typo than a new word, but he hasn't copped to it, so ya never know.