It's just... so peak ðŸ˜
I see that they're not dwelling on why Lapis plans the purge. Us audience can infer, but Natori here truly just... cherishes Lapis, despite what Natori feels are self-serving motivations (staying alive and working), so she stands by Lapis, right or wrong. I feel that when her reasons are revealed, Natori might thread the needle to a better, third path tho.
As for Lapis' side of the night, fondness for Natori's support, reckless acts and sheer presence wars with guilt at using her. As Natori confronts her, perhaps Lapis comes to feel that she can't stand Natori not by her side.