well, notice how neither obi or yoda say "kill him" directly.
(this is a whole other topic, of which i have seen MONSTER size threads)
the closest either say is when luke says "i cant kill my own father" and
kenobi says "then the emporer has already won"
other than that, there is no talk of killing him.
vader "obiwan once thought as you do"
what if obiwan never stopped thinking that?
remember that obiwan never tells luke what to do,
and he never gives him the answers. "you must do what you feel
is right of coarse." at no point is luke given any orders.
they suggest, but they dont demand.
see, otherwise my problem is this:
1-they know anakin is the all powerful chosen one
2-they know that palpatine is the most powerful dark force, besides vader.
3-luke is inexperienced, as powerful as he is
so, did they really expect luke to be able to kill vader and the emporer single handed? (hehe...unintended funny) if they wanted to simply bum-rush the throne, why not train leia as well...two lightsabers are better than one, right?
i dont think they sent luke in alone, to kill the chosen one.
i think ben knew that anakin was the only one powerful enough to kill sidious.
yoda, i think, had faith that luke could have the power to do so as well.
as much as yoda b!tches at luke, just look as his face when luke manages
to lift his xwing with such limited training. his eyes damn near popped out of
his head. he could not show his amazement to luke though, as it would feed his ego...which is bad.
so i believe that they at least HOPED that vader could be turned.
i think ben had no faith that luke could kill the emporer...only anakin, only the chosen one.
whereas yoda felt that even if luke were to fail, with the help of his sister he could still have the power to turn back to the good side and do the deed.
imho anyway