Obi-Wan's mercy is not only appropriate for a true Jedi, but leaving Anakin/Vader alive supports the interpretation that Luke was viewed as the "last hope" for something other than killing Vader. Killing Vader does *nothing* to usurp the Emperor. Y'know... the most powerful bad guy in terms of both the Force and political influence.
If you think about it, having someone as powerful as Vader turning on Palpatine is really the only way to actually win.
People will keep pointing to the exchange when Luke tells Kenobi that he can't kill his own father and the response is,
"Then the Emperor has already won." But that sort of thing is for structural storytelling purposes. If you let the audience know ahead of time that Luke isn't actually meant to have a to-the-death duel, you'd lower the tension/suspense for the movie's climax.
At no time did either Yoda or Kenobi tell Luke that he had to *kill* Vader. It was essentially variations of "you must *face* Vader." Having a long game of 20 years for Luke Skywalker just to be trained to be a more effective killer doesn't jibe with his actual training, and would make no sense. The Dagobah cave failure would make no sense. Yoda's lessons about "never attack" would make no sense.
A Jedi should kill when there's no other choice. Luke was trained to be able to defend himself, and yes, kill if he has to. But the idea that killing Darth Vader was the ultimate goal and that 20 years of Imperial terror would go by before Luke starts being turned into a better killing machine than Obi-Wan was 20 years prior (when he *defeated* Vader) is, well... kinda dumb.
