He was never going to do quite what they thought he was. He would kill Iris West, an Iris West. They couldn't see past the illusion of her death. He already knew Barry would believe it, would fear it far too much to question what he saw. He remembers how it feels, the creeping grief consuming him each day. Savitar still believes in destiny, but destiny is a bit more complicated these days, not confined to Central City. It spans millenia, it spans dimensions.
He wants what was taken from him, he wants her – so he creates her. Another time remnant like himself. Iris West - the Iris who never left her own timeline - she remains in the dark about his plan, not able to tell Barry. Her last words, of love, are Barry's alone, he lets him have that. But the other Iris, her words, all her words, are his forever more, whatever they may be. His Iris is safely ensconced in the speedforce where she battles futiley to escape. No rescue is coming for someone no one else knows exists.
It always had to be this way because her death creates him, because it breaks him. That broken Barry is so desperate, so driven to banish him to the one place he needs to be. It's meant to be a punishment (it still is).
She pleads with him, pities him but he is intent that she will love him again in this eternity. An eternity that they will share. He tries over and over. Expecting a different result is madness, he vaguely remembers the quote. He's gone mad before, it isn't any more of a pain to live this again than he has had before. The hope she will love him back squeezes into him, a tiny speck of Barry Allen magnified. He'd never had hope before because he'd been alone here. It's hope, but it's crushed over and over by her refusal to accept him, her rejection in every moment repeated endlessly.
Has he made his own hell? Only time will tell. His one comfort is they all hurt in parallel. Every Barry Allen broken. Every face that looked upon his face and rejected him, has fallen, scarred by their anguish that his prophecy came true. Their hope was their downfall (just like him). Their arrogance bathed in a belief that could never be greater than his own godly ambitions. His might, his knowledge backing him, infinitely more powerful than the pathetic people bracing his naive self for this future.
If he can't have love, can't extinguish that feeling himself, he'll turn it into hate and make them truly hate him in return. At least then they will feel something, unable to turn their backs on him again. He might have been disposable but he's unforgettable, always in their hearts.