He was transhuman, technically. When he was Anakin, he only turned to the Dark Side because he believed he could use it to save his wife. However, once he turned, it changed him. True, he changed back but that does not discount his original "fall".
You recall where he killed younglings, right? Through the franchise, one can not turn to the dark side without doing what most in the non Star Wars Universe consider actions of evil. Kylo killed his father Han, which logically would have helped to strengthen his bond with the Dark Side?