^It's funny, you say "Selfless" doesn't portray men as demons as women as angels when, in fact, a female demon took the choice to murder a room full of men. I'm not defending the actions the men took to get there, but Anya made a choice, one that she was rapidly forgiven for.
She was forgiven because the deed was erased, because she was willing to sacrifice her own life to erase it, and because she turned forever from that path at that moment. In the absence of those things Buffy was willing to kill her.
You can only judge people by what they do, and she never hesitated to inflict suffering upon men because Judge Anyanka decided they were in the wrong. We never even got a decent explanation as to why she only went after men. The other Vengeance Demons didn't.
D'Hoffryn recruited her for that purpose:
Anya: "What would I have to do?"
D'Hoffryn: "What you do best. Help wronged women punish evil men."
D'Hoffryn: "But only to those who deserve it."
Anya: "They all deserve it."
D'Hoffryn: "That's where I was going with that, yeah."
Given the immediate context of Olaf's betrayal it's not surprising that Anya would be amenable to this idea, nor is it surprising that, once assigned and accustomed to a niche, she would stick with it. D'Hoffryn's a smart (and hilarious) guy, folks who are personally motivated to do their job tend to be more reliable and dedicated than those who aren't. Anya alleges that Halfrek's speciality is "bad parents", you think she might've had an unhappy childhood?
Once she joins the Scooby gang and hooks up with Xander she discovers that all men don't
deserve it. Once she returns to being a vengeance demon she has great difficulty in fulfilling her role, her murder of the frat boys comes in response to heat she's receiving (from Halfrek and D'Hoffryn) for not pulling her weight in the vengeance department. She tells Willow that they deserved it, but she's not trying to convince Willow of that, she's trying to convince herself; she knows it isn't true.