Because nobody on your side has ever killed someone? Come on, Quentin.

[ julie rolls her eyes, reaches for the bottle. she's starting to understand how this is her fault, how she encouraged and nurtured something in all of them that turned them into monsters, and she needs quentin to understand that that's a significant part of who susie is. ]

It's not about good or evil, it's about the fact that what we are runs deeper than one awful thing she was part of.