Who's slandering?
You slandered Newby, insulting him and his relations. I guess it isn't really slander though, because slander is false
and injurious, but what you said wasn't false.
But here, a few reasons we should kill Newby.
If a man marries both a woman and her mother, it is wicked. Both he and they must be burned in the fire, so that no wickedness will be among you. If a man has sexual relations with an animal, he must be put to death, and you must kill the animal.
EDIT -
Interesting, Leviticus 20 says God will punish the innocent.