You must kill Kirk to get his armor set. NOT killing him will not get the set. Then, if you kill Deacons of the Deep, the opportunity to get the set will be lost until next game cycle. Kill Kirk, go to Rosaria's Bedchamber, and the armor set will be on a corpse on the right side of the room (facing Rosaria) near the gate in the back near Rosaria. If the set doesn't appear, try warping to Firelink Shrine and then back to Rosaria's bedchamber.