Knowing their background does not make them justifiable but understandable,
Certainly! After reading their backstory in manga, I somehow cried because what they experienced as children were ruthless. But them experiencing a traumatic childhood doesn't mean that what they've done were permissible. I'm still looking forward to how Togashi-sensei will end up spiders because I somehow had this bad feeling that the original ones in spiders will be all killed ಠ﹏ಠ (hopefully not). I'm still hoping that Togashi-sensei will somehow give Chrollo and other spiders a chance to change the course of their fate (even though I know Togashi-sensei isn't fan of giving second chances (ب_ب), still I'll be hoping).