"Scandal" kept its promise by revealing Vargas' killer in "Extinction" which aired March 16. [SPOILER ALERT!] Rowan a.k.a. Papa Pope was the one who killed the President elect. It wasn't that surprising since he had been the biggest suspect, but there's a bigger story behind that.
It turned out that Rowan killed Vargas because the Mystery Group that Zoe Perry's character works for held his old flame from grad school, Sandra, hostage. He had to make sure Mellie became president or Sandra's dead. So when Vargas won the election, the group ordered Rowan to kill Vargas and framed Cyrus for it.
The group refused to release Sandra because she's Rowan weakness. But Rowan wouldn't let himself be controlled by anyone or anything, so he killed Sandra himself. "I have no weaknesses," he said. "No one owns me." But he's wrong because the Mystery Group has his other weaknesses, his daughter Olivia, and he's pressured to make sure that Cyrus goes to jail and Mellie becomes president.
In a preview for the next episode, Olivia finds out that her father's behind the assassination of Frankie Vargas. She makes a shocking decision and asks Huck to complete one of the most difficult tasks yet, killing her own father. And somehow, Huck and Quinn are forced to pull their guns at each other.
"A Traitor Among Us" airs Thursday, March 23 at 9/8c on ABC.