A so-called "Targetted killing" is an extrajudicial assasination. It is illegal unter International law and would constitute an act of war. I could have unpredictable blow-back, including the possibility of Iran's direct involvement and it would be a stick in the eye to Vladimir Putin who has found a dipolomatic solution to avoid a US-initiated military intervention.
Russia calls for international cooperation to fight ISIS (Reuters)
from the article: Time for a targeted killing in Syria