WHo says a non-Jedi couldn't. This isn't a representation of the movies here but based on the d20 System PnP game. I played a nice HK-47 type droid named Halberd, though built more for fighter piloting than face to face combat, and during the course of the game (we were going through the Star Wars timeline with our characters instead of the mains) and I killed Darth Vader in the Death Star run while another PC did the bomb drop.
In the RPG non-Jedi can take out force users, especially on a critical hit.
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Somehow I doubt it that you wound be alive to face 3 Sith Lords (which requires that you have slaughtered already an army of Sith and Dark Jedi) let alove kill/destroy/turn them.