It shouldn't! It's connected with the mind melding that all protoss do between each other. Since his Meister ability is different to his psionic one he doesn't have a 100% grasp on it yet, and since he knows this he always holds himself back because he can't yet fully control it and doesn't want to hurt/kill anyone because of it.

Basically Tassadar can imprint hallucinations into the minds of othes by using his own memories, but he can't pull his mind away from the target's mind yet or keep it separated - which tends to allow the link to go both ways, so he takes on the frame of mind of his target. So in Karkat's position, being violent and blood thirsty >.> so it would need to come down to either a partial loss of control (someone pulling him out of it), or Karkat (or Tassadar) either being knocked out or withdrawing from the fight.