If their "sole purpose" is to kill men, where does killing other species fit into that? Or do you believe the children are men?
Their original purpose was killing humans, they probably gained sentience beyond what the Children could control, and turned on them. It's a well worn trope.
This is to the larger conversation at hand, about King's Landing and/or the political sub plots....did you guys forget this image from Bran's vision?
Winter is coming to King's Landing. I know some people interpret this as "Snow on the throne" but no other vision he's ever had has been symbolic. It's all stuff that has happened, or will happen.
Fred.