And strategically, assassinating Snowden is about the worst possible move for a US linked agency since (i) it makes them look bad, even to some of the people who think Chelsea Manning got off lightly and (ii) it makes Snowden's revelations interesting to a whole bunch of people that don't read Hacker News and think that articles about email monitoring sound incredibly dull.
You don't have to believe that every US chain of command in every theatre of operations has qualms about "disappearing" people suspected of quietly leaking secrets to think that causing an international incident over a public figure like Snowden, even if the assassins were guaranteed to succeed and not perish in a firefight with Russian cops called by an FSB security detail, isn't going to do them any good.