Board Thread:Episode Discussion/@comment-24573226-20170204043226/@comment-26397825-20170205053527

They've seen the worst in humanity - Mercury is an assassins kid, and Emerald is a street rat. Hurting or killing others - especially people who severely injure you - is different to literally watching the birth of destroyers of humanity who society has told them is something they should fight against whenever possible.

It's the same as killing an animal for food vs killing a person in self defence. They're both for your continued surivival, but one has a significantly higher impact on your psyche than the other.