First of all I would check Player [y] NLR timer, if it was over, I would teleport Player [x] to some quiet place and ask him for proof. If he provided me with enough proof, I would teleport Player [y] to the same spot and ask him about his side of story ( And if he has got any counterproof to give ). If it turns out that Player [x] randomly killed Player [y], I would give Player [x] (RDM) a regular warning and Player [y] (NLR) verbal warning, otherwise I would warn Player [y] for breaking NLR. And once again if I won't be sure about it, I would ask admin to assist me.