twinsies!! Okay but like on the one hand she knows Lust is a terrible shitty not-person that kills people for the hell of it sometimes, but on the other hand, who isn't a terrible shitty person-or-not-person that kills people for the hell of it sometimes? She dated Eridan for like two years she lived with two different serial killers half the people she befriends go SURPRISE, DEAD PEOPLE MY BAD, she no longer has any place to judge. But despite... that... she's an interesting enough woman to talk to and Ruka has gotten zero indication that Lust has any inclination to have her killed! So she's pretty willing to remain friendly ("friendly"), and that's good enough.

The fact Lust literally could not give a shit about Ruka's emotional problems is actually a positive. The fewer people who care about that, the better!