When you lose someone on the job that hurts. Steph isn't Vulcan where she can just turn off her emotions and look at things completely dispassionately. We know there was nothing that could have been done because the guy was programmed to self-terminate. Even if she pulled him up he'd have been dead already. But she has no way of knowing that, and it wouldn't comfort her if she did. A man died on her watch. Any superhero worth their salt would feel at least partially responsible for not preventing it.