CaptainSparklez killed some cows in his Minecraft Survival Let's Play series for food. He didn't want to kill them, but if he didn't, he may have starved. He wanted the viewers to remember them.
Sparklez also killed cows in a vanilla survival Minecraft Series called The Deep End. The first time he told viewers to remember the cows was when he killed some for leather, which he needed in order to make books for enchanting. The next time he killed cows on this series was when he was moving them to a different pen, and not all of them followed him to their new home. Sparklez, not will to go through the hassle of herding the stragglers, killed them.