system
Given the following post, respond as if you're commenting on Reddit:
human
[SW ROTS] What turned Anakin from "Your friendship means everything to me." to "This is the end for you my master" in a couple of hours at most?

Yes the Dark side etc but really? A decade long friendship over in a few hours and for no real reason. That's gotta be the quickest turnaround in history.
gpt
As with all things, the (sadly) non-canon Revenge of the Sith novelization humanizes it much better.

You have to understand that Anakin is super fragile, emotionally.  When Padme tries to tell Anakin that she's pregnant, his mind immediately jumps to "Has she changed her mind about me?  Is there someone else?" and the rage builds before it's obliterated by her revealing her true secret.  He does this several times with her, and his trust in her frays and erodes just as everything else about him does.  He describes his life as a free fall in the dark.

Add on to that, the Jedi are proving untrustworthy, even to the point where Obi-Wan Kenobi, the most trustworthy guy around is using Anakin to do their dirty work for them by asking him to spy on the Chancellor.  Through it all though, Anakin really means what he says to Obi-Wan the last time that they meet as friends.  That friendship does mean everything to him.

Which is why his fury is unstoppable when Padme arrives on Mustafar and all her suspicious scheming and sneaking and *politicking* comes to a head when she starts telling him about these awful things Obi-Wan has said about him.  Anakin tries to brush it off.  "I don't want to talk about Obi-Wan anymore", but it's soon crystal clear, to him, that Obi-Wan has poisoned Padme against him.  She won't, can't, understand where he's coming from, the things he has had to do to keep her safe, to keep her alive.

And then Obi-Wan himself steps out of *her* ship.

And Anakin can no longer control himself.

The Jedi betrayed him.  Treated him like an unclean thing stuck to the bottom of their boots, but they used him none the less in underhanded and traitorous ways.

Padme betrayed him.  She refuses his offers of a better world, a better future.  Their love?  It means nothing.  Their upcoming child?  She'll take it away.  Her politics means more to her than he does, and he sees that now.

But Obi-Wan?  Anakin could have never expected Obi-Wan to betray him so much.  Even when doing the council's dirty work, Obi-Wan hadn't exactly been *happy* about it.  But here he is, on Mustafar, at the site of what should be your greatest triumph, with your wife who is now saying she can't be with you anymore, and it's all because of the stories that *he* told her.

Obi-Wan's friendship meant everything to Anakin Skywalker, and the perceived betrayal of that friendship turns Darth Vader's heart into a furnace of fury.  With one last controlled effort, he offered his friend, his *betrayer* the opportunity to walk away, to go 'meditate in a cave somewhere'.  But Obi-Wan does not, and the last traces of Anakin Skywalker burn away in the furnace heart of Darth Vader, and nothing will ever extinguish it until Obi-Wan is dead.