It's the final few minutes of Revenge of the Sith. The Star Wars saga, after almost 30 years, is about to conclude for us here on Earth, but for our characters though, there is plenty yet to play for.

Obi-Wan and Anakin are nearing the end of their duel on Mustafar, and after an arduous battle atop the flowing lava fields Obi-Wan has leapt from his hovering platform and taken higher ground.
Anakin, fuelled by rage, determination, betrayal and murderous intent slows and glowers at Ben.
"It's over Anakin. I have the high ground." Ben gestures to his higher position.
But pause and think about the next scene, and the thinking behind it. In fact, go back to about 20 minutes before.

Anakin has turned to the dark side. Devastated and horrified, Ben begs Yoda to allow him to face the Emperor. But the Jedi Master says no. Yoda will face Palpatine, and Ben will hunt down Anakin.
Hunt him down to kill him, to end the threat he poses to not only the Jedi but to the galaxy.

Fast forward to Padme's balcony. Ben visits to gently question her, and to inform her of Anakin's fall. Padme, in denial, refuses to help and as Ben departs the truth dawns on her.
"You're going to kill him, aren't you?"
Ben cannot deny it - it's the truth. After asking about the identity of her childs father he looks sadder than he ever will again in the saga.
"I'm so sorry."
Kenobi, the man who practically raised Anakin since age 9 and into adulthood, now carries the burden of having to end his young padawans life.
He has essentially become a Jedi assassin. Perhaps the long years in isolation not only gave him chance to watch Luke and train with Qui Gon, but to pull himself back from the brink of the darkside himself...

Back to Mustafar. Ben knows Anakin inside out. They know each other, having fought alongside each other as equals during the latter days of the Clone Wars. Ben knows that Anakin's arrogance and pride, and knows that in his current state Anakin will not be able to simply leave the fight alone.
But Ben is there to kill Anakin. "He has become a very grave threat." Ben said it himself. And he knows that his "High ground" line would seem like a taunt to Anakin.
And Darth Vader fell for the trap, just as Obi-Wan planned.
Something Yoda couldn't have accounted for was Ben not killing Anakin entirely. Up until this point Kenobi had been a model Jedi, the go-to man of the Jedi Order. But when it came to the crunch 'attatchment' as Yoda put it, or 'unconditional love' as Anakin may have worded it, stopped Ben from finishing his young charge.

Here's an alternative idea. What if Yoda had heeded Ben's request and allowed Ben to face the Emperor. Kenobi would certainly have perished, but look at the other situation. Yoda would have faced Anakin.
Sideous himself said that "Darth Vader will become more powerful than either of us." And Yoda knows full well of Anakin's skills, weaknesses and potential. Yes, the Jedi Order didn't quite know what to do with him, or how to best contain him. The Prophesy was turned inside out by the actions of that day.
But Yoda would have almost certainly prevailed against Anakin, and that threat would have been ended. After all, in the long term, what was the bigger threat? Vader potentially overpowering his master Palpatine and taking the galaxy as a fit, healthy Sith, or an ageing Emperor with no apprentice ready to stand by his side? After all, he'd sacrificed Dooku in order to gain Anakin from the Jedi.
Yoda could have taken Anakin, leaving himself prepared (and that's the key word, Yoda was not prepared for his clash with the Emperor, it was sprung upon both him and the Order - Palpatine had "waited a long time for this moment.") to clash with the Emperor and possibly defeat him.

But that didn't occur. Yoda fought Palpatine to a draw and fled into hiding, and Kenobi couldn't bring himself to complete his mission, presumably leaving Anakin to die on the lava shores, and consequently threw the galaxy into even more darkness.

On that Mustafar platform Anakin wasn't there to kill Kenobi. Ben could have walked away and lived, at least for the time being, until he'd pushed Anakins buttons and lured him into conflict. But Ben was always there for one purpose - to end the Sith threat and eliminate Skywalker.

Unfortunately for the galaxy, he didn't quite have the stomach for it.