But how would Koth know this? You are judging him as the player sitting on the other side of the monitor. Even Senya, his mother and a Knight, didn't know what really happened to Thexann until Vaylin told her at Asylum.
Let's suppose Lana took Koth to Ziost during the intervening 5 years, to show him what happened there. What tangible proof is she going to offer that Vitiate, a disembodied world eater, is the same as Valkorian, the man Zakuul has seen sitting on the throne for what, centuries? I hope that part of Koth's story is coming to grips with this, but I can understand why he doesn't automatically believe Lana or the outlander. And really, it doesn't seem like Senya really believes them either, but for whatever reason Koth gets a lot more of the hate for it.