"Children...Ah! Killing them is actually a great favor to mankind and for each person on the face of this planet, especially when those damn "cute little f--king faces" give false hopes to the humanity. I love to kill them, they are more attractive dead. I'm not the "villain" here, I'm just ridding the world of these pests."
"This DNA bead came from a little girl in Riverside, California - didn't like to wash her hands. Took me three whole weeks."
For many the killing of people who are 12 years old or younger is one of the worst possible things you can do, it also goes against the very concepts of what we see as basic humanity (even if the murderer is a child too, or if the child was a villain themselves since villains who are children are much easier to rehabilitate) - for this reason villains who are portrayed as willing murderers of children are reviled far more than those who kill teenagers and adults: in the past the killing of children was an extremely taboo subject matter in fiction, though it has become more common (though no less controversial).
This is not to be confused with a villain that has murdered their offspring (as they are a Family Murderer instead), though it is possible for these two categories to overlap.
NOTE: Please do not add characters who only kill teenagers and/or adults to this category, as teenagers and adults do not count as children. Also, do not add those who have attempted murder on children and failed, as they are not to be counted for.