She's doing it while smiling pleasantly. Devils are devils, they are what they are supposed to be, evil demons. Humans, however, should possess empathy and be understanding of others' circumstances. There are people lacking those qualities, but they are considered psychologically abnormal, sociopaths or at worst psychopaths. The real life monsters. Even when the society was underdeveloped and ruthless, like the Middle Ages, an executioner was considered to be a dirty job.
Other than that, Makima is stronger than most devils, I'd say, with a power like that. Strong like a monster.