Hence my desire that something keeps Obi from killing him. Like the idea above about others coming to save Vader. That would humiliate him and make him hate Obi even more.Killing isn't an evil act, in and of itself. MURDER is. Like I said, old school. @Stalker0 has also made a very valid point on the matter.
If you can't accept either of those, then how about, "Because Vader has to be alive in 'A New Hope'"?
Don't both Obi and Yoda tell Luke he has to kill Vader? Or am I recalling that wrong?