[Leans back from his communicator at what Karkat says, this must be some kind of practical joke that isn't funny to him at all but must be hilarious to everyone else, but why would Karkat do that to him? That sounds like something a mutant freak like Sollux would do, not Karkat. Either way he's feeling very uncomfortable about his sudden momento mori, the urge to throw his communicator across the room and run from it rather strong in his instincts.
Combine all that with the realization that Karkat doesn't remember the last time he was here, just like Vriska never did causes him to accept defeat in trying to get Karkat to remember anything from last time, what was the point anymore? Even though he accepts that, he doesn't want to accept that if he leaves this prison he's going to die and that's all there is left to him.]
That's --
The worst fuckin' practical joke I've ever heard, as if I would die anytime soon!
[Can anyone really blame him for being upset and immediately denying the truth?]