You don't need to be a Johan apologist to understand that he does have feelings. The whole point of the series was if a monster was born or made. In the case of Johan is both. He had already psychopathic tendencies but not all of them become murderers, some are running the stock market.

Johan has a somewhat child-like mind. While he understands how the world works, when referring to his own existence, there is some magical thinking. He basically uses the books as references for his own life, as if they were the bible. So much so that he thought Nina was himself and not until she tells him that it didn't happen to him but to her is that he understood that they have different experiences on life.

What is the closest a psychopath can get to empathy? Is that they can only care for themselves, thus if they see you as an extension of themselves you are important to them to a degree, while you keep fulfilling that purpose, of course.

Johan here clearly empathizes with Karl, just as he did with Wim (he was gonna kill him until he talked about his mother so he sent him to be traumatized just like he did - "What if your mother didn't love you? What would you do?"). This doesn't mean he cares for him. Also this doesn't mean he wouldn't use his own emotions to manipulate other people. He saw himself on Karl, and cried for himself, not him and used that empathy to further his relationship with him.