To the best of my knowledge pwn is a derivative of the word "own" used for the purpose of celebrating or spreading word of one's skill or achievments.  The situation you describe really would not fit that...you cheap-shotted someone who was already in a fight and then proceeded to take out the mob he had already almost killed.
I respect your right to gank someone fighting any mob worth fighting over (rare spawns, world bosses, etc.).  You both rolled on PvP so its fair game and honestly a good tactical decision.  I applaud you for showing sound judgement and good strategy that minimized the risks to you while gaining you a decent gear advantage in adddition to the HK.  But until you defeat someone (player or mob) easily or in a fantastic manner in a fair fight please don't come here saying you "pwned him".
Saying you pwned someone in that case is like bragging about being able to take down anyone in the ICU (Intensive Care Unit) of the nearest Hospital.