Are you suggesting it is likely anyone thought Vader would just stop? How about detaining him instead of killing him? For the story, he could escape....
Luke?
And, again, how on earth would you detain him? Obi-wan has no power here. He's a rebel. Heck, at this point, the Rebellion has pretty much zero power. Someone who can literally rip apart a spaceship with the power of his mind? Never minding that Obi-wan has a duty to protect Luke and Leia, not stop Vader. The longer he stays with Vader, the greater the chance Vader learns of Luke and/or Leia, resulting in Obi-wan failing yet again.
If Obi-wan kills Vader, the Emperor wins and millions more will die at the hands of a virtually immortal Sith lord. After all, without Vader's weakness for his children, the Emperors new apprentice will be unstoppable and certainly will have no reason to betray the Emperor outside of standard greed.
This is a bog standard morality tale. Again, it goes back to the quote from LotR - “Many that live deserve death. And some that die deserve life. Can you give it to them? Then do not be too eager to deal out death in judgement.” You might not like it, but, the story here pretty plainly supports this interpretation. Kill Vader now and evil wins. Do evil, even with good intentions, and evil wins. That's been the basic premise of Star Wars since day 1.
I'm really frankly baffled how you can interpret Star Wars any other way.