He tells you this is a future. A future yet to be. This is how he saw what was going on in the games. He was given this by the Queen just as a bargaining chip to get him to do this. But he used it to see into the future. He tells you that he saw it all. All possibilities, all futures. All of them, failures. This plan didn't work. How could it not? He tells you he thought.
But then, he explains, he thought about it like how his supposed mentees would think. So, he analyzed the situation and came to the conclusion that he wasn't the one to kill the White Queen. In fact, none of the Carpascians that were selected by him were supposed to kill any of the monarchs. So, when the other competitors started finishing the gauntlet, he had no choice but to kill them. He shows you the closet where he hid the bodies.
He tells you that in order to come to a conclusion that made any sense in the world, he had to pick three of the stupidest, laziest, oil and water trio he could come up with, because that's how this game works in the end, irony at its finest. So that then leaves the matter of whom which of you is to kill. He said he looked at all the permutations and he found that he wasn't even supposed to be in the killing group. Instead, in his place, the Derse ARCHAGENT. And he would kill his own queen and you would yours.
You leave his room filled with a mind of conspiracy. Plots that nobody is to be shared.