I have no dog in the fight. But what really matters to Hawks is how well Kempny was playing as a Hawk. How well his is playing for the Caps now, doesn't really influence the trade at the time.
To take this "why to trade this player" to another level. Leafs traded Kessel for Kapanen and 1st rounder. Hardly a good return for what turns out to be a very good player with the Pens. But at the time of being a Leaf, Kessel had morphed into a average to poor player....yes despite his numbers. For whatever reason, he was playing poorly, Leafs had to jettison him and his 8m contract (6.8 with retention). The fact the Kessel found new life in Pittsburg....maybe the only place it would have happened, doesn't make the trade bad for the Leafs. It just had to happen considering the circumstances four? years ago.