The Galactic Empire is only as powerful as it is because of Obi-Wan Kenobi.
[spoiler]You’re Obi-Wan. You’re locked in battle with your best friend, your brother, your pupil, someone closer to you than family. He’s just choked the hell out of his wife- his PREGNANT wife, and, earlier in the day, he slaughtered a whole bunch of CHILDREN, not to mention the head of your religious order. Your ex-pal has established himself as one pretty bad dude. He’s admitted to being a member of an organization whose existence you’re sworn to frickin’ DESTROY. So what do you do? You fight him. I’m with you so far.
So you’re fighting, and he’s ranting, and he’s pretty much reaffirming the whole evilness thing he established, oh, by choking his frickin’ pregnant wife right in front of you. So you fight him to the point where you’ve got the high ground, he can’t win, and you, in all your Jedi sportsmanship, give him the chance to surrender. Mighty big of ya. He, of course, does not. You know what he’s gonna do, you warn him not to, he does it anyway…
and you cut off his LEGS and leave him in a heap next to a river of LAVA. He then catches on frickin’ FIRE, and still he screams his hatred for you. So what do you do? You speechify and then walk away.
Whiskey. Tango. Foxtrot.
He’s a frickin’ Sith! He’s wicked powerful! Plus, he was your best friend YESTERDAY! If anybody deserves a quick death, it’s him. And what do you do, Jedi jerk? Leave your pal there to burn horribly to death. Or, turn your back on your greatest enemy, without finally defeating him.[/spoiler]
It’s either a jerk move or poor strategy.
Vader went nuts on the galaxy for the next tweny-odd years, choking people, quashing rebellion, and just being a telekinetic jackass on a respirator. And you could have stopped it either by being a good friend or by being pragmatic and finishing what you started.
But you did neither. Nice work… jerk.