Given that the Others’s motivation is “extinguish everything we would call life,”” and that they “hated iron and fire and the touch of the sun, and every creature with hot blood in its veins,” I’m going to guess they killed them like they do all warm-blooded creatures.
This has been another PSA that no, the White Walkers are not good guys, the Night’s King is not evidence of a peace treaty that ended the War for the Dawn, stop it already.
