Under normal circumstances, when a person kills another human, they are supposed to feel some form of guilt or fear. Even cold-blooded killers feel something, even if it's just for a brief moment. Killing a fellow human is too cruel in so many ways.
So, when someone kills without feeling anything at all, that person is often seen as emotionless. People like that are called demons, psychopaths, sociopaths, or monsters. They have no attachment to anything, no remorse.
Klaus is one of those people, but his case goes beyond that. When he first killed Matin Guan, he expected to feel something—regret, guilt, anything. But after wiping the guy out, Klaus felt perfectly fine. In fact, he felt happy.
He doesn't know where that part of him came from, but he never cared. He craved it—the bloodshed. A part of him needed it. After his first encounter with Matin Guan in Sunlight Forest, something inside Klaus wanted more.
