Power corruption and lies

Well, the reason Darth Vader killed only Obi-Wan is because everyone else ran. This is actually the first time, as far as I can remember at least, where the lower level PCs are owning up to it and running. Not a horrible strategy, that.

The times I've killed everyone have been for storyline's sake. It's a DM's trick I've used before in table-top games. Often the heroic narrative involves a trek to and back from the underworld (e.g., The Odyssey, Vampire games, Chrono Trigger).

Don't forget that in most campaigns, although the main villain somehow becomes super-powerful relative to the PCs, he usually dies. Generally right after becoming super-powerful, or as a consequence. The moral is blah blah blah power corruption and all that.