Moo-rderer is an achievement/trophy you can earn in the Witcher 3’s expansion, Hearts of Stone. To get it, you’ll have to engage in some bovine slaying on a mass scale (vegetarians need not apply).
Once you’ve bested your bovine foes, meditate for an hour to spawn new cows. Slice, dice, and repeat until the game notifies you that you’ve earned the trophy (after 20 cow kills). You can celebrate by munching on some beef.