Vincent's NT Word Studies
21. Betrayeth (paradidontov). The present participle: is now engaged in betraying.With me. "He does not say with you: thus separating the traitor from the rest of the disciples, and showing that now he alone has to do with that wretch, as with an enemy" (Bengel).
Robertson's NT Word Studies
22:21 {That betrayeth} (tou paradidontos). Present active participle, actually engaged in doing it. The hand of Judas was resting on the table at the moment. It should be noted that Luke narrates the institution of the Lord's Supper before the exposure of Judas as the traitor while Mark and Matthew reverse this order.