They confronted Kaolinite and she turned into a Daimon and attacked them. It's not like they knew her backstory. And for all they knew, when she turned into a Daimon, that was all that was left of her (manga and in extension, Crystal, is a little more cynical than 90s anime in regard to death or redemption).
I can't really speak for Beryl since the idea of past lives influencing present is shaky grounds for me. But for her death in the manga, it's simple. She was killing Sailor Moon with her hair. Sailor Venus tried to free her, but cutting the hair didn't help so she did what she had to in order to save her princess.
And as to taking law into their hands, again supernatural beings threatening the world kinda falls out of the usual jurisdiction.