It's true that he killed Sara. Slade had tortured him. Branded him with Shado's tattoo to insure he would never forget what he did. So that Oliver would never forget why Slade tortured him.
But Slade was Oliver's last friend. The only friend in the world. Tommy may have been his best friend in the past, but he would never do what Slade had done for Oliver. Laurel may never forgive him for saving her sister's killer. Detective Lance would never forgive him. But they would never understand Oliver's relationship with Slade.
They'd never understand the trust the two of them built. The courage Slade must have had when he first allowed Oliver to live. The live and death experiences the two had would never compare to the miniscule time Oliver had shared with Yao Fei, Shado, or even Sara.
To Oliver, Sara was still same person that died on the Queen's Gambit. He saw her die and the fact that she's here still hasn't sunk into his conscious. He could betray the last few day's memories of her over the one good friend he still has.
Despite Slade electrocuting him and branding Oliver with Shado's mark Oliver understood that the mirakuru only enhanced what was already there. The rage that Slade had at Oliver was Oliver's own fault and it was time he manned up on that. If Slade still wanted to kill Oliver after he cures him, then so be it.
"Don't die on me yet Slade." Oliver prays as he administers the cure into Slade's bloodstream. You're all I got now. Oliver still had to get himself and the unconscious Slade off the burning Amazo too. Well there's only one way off since Anatoly blew the damn place up. Not that Oliver could blame the guy.