Can Weaponmaster stop me from needing to kill a unit with Blade of the Ruined King?

No, Weaponmaster cannot stop you from needing to kill a unit to equip Blade of the Ruined King.

According to RiftJudge FAQ #5147, while the Weaponmaster keyword allows you to pay the equip cost at a discount (reducing the power cost by [A]), you must still pay all other portions of the cost, including non-power requirements like "kill a friendly unit." Weaponmaster only reduces the power [C] requirement of the equip ability; it does not bypass mandatory sacrifices or other non-power costs.