Obi-Wan & Yoda didn't kill Darth Vader because:
Obi-Wan and Yoda didn't kill Darth Vader because Darth Vader was the only one that could kill the Emperor.
*No you're wrong man, cuz dude, cuz like, like Obi-Wan was too old to beat Darth Vader dude!
CORRECT...BUT...OBI-WAN WAS OBVIOUSLY YOUNG ENOUGH TO DEFEAT DARTH VADER IN EPISODE III, SO IT SEEMS LIKELY OBI-WAN COULD HAVE DONE IT AGAIN A WEEK, A MONTH, A COUPLE YEARS, A DECADE LATER...but he didn't.
*Well ya ok man, but like, but dude Obi-Wan couldn't get close enough to Vader!!!
OH REALLY? WELL LET'S SEE...OBI-WAN KENOBI, AT THE AGE OF LIKE 50 SNUCK ABOARD THE DEATHSTAR, RAN AROUND WEARING JEDI ROBES, AND MANAGED TO TURN OFF THE TRACTOR BEAM COMPLETELY UNNOTICED BY THE HUNDREDS IF NOT THOUSANDS OF STORMTROOPERS ABOARD...One would think if he could do that at 50 he could do it at 30.
*Ya but come on dude, Obi-Wan kept egging Luke on to kill his father dude, Darth Vader!!!
BUT LOOK...LUKE NEEDED THE INCLINATION TO KILL HIS VADER, HE NEEDED TO GO FAR ENOUGH TO DEFEAT HIS FATHER AND CHOP OFF HIS FATHER'S HAND AND LEAVE HIS FATHER COMPLETELY DEFENSELESS...AND THEN AT THE LAST MINUTE SHOW HIS FATHER COMPASSION...AND REMIND HIS FATHER THAT JUST BECAUSE YOU ARE MORE POWERFUL THAN SOMEBODY DOESN'T MEAN YOU SHOULDN'T SHOW ONE MERCY...Anakin needed to see the Emperor offer Luke what the Emperor offered Anakin, and watch Luke turn it down. Anakin needed to see this in order to turn back from the darkside.
*But dude even Yoda said Luke needed to kill Vader!!!
YODA SAID 'CONFRONT' VADER, NEVER SAID KILL HIM. AND OBI-WAN NEVER TOLD LUKE TO KILL VADER EITHER.
*But duuuuuuuuuuuuuuuuuuuude!!!!!!
OK YOU WIN.