There are three known firekeepers in the game: Lady Darkling (the Darkmoon firekeeper in Anor Londo), Anastasia (the firekeeper at Firelink Shrine), and Quelaan (the firekeeper for the Chaos Servant covenant).
It's known for a fact that Quelaan needs Humanity to survive, which is why her servants bring it to her. I doubt we've been alone in thinking that this is simply due to her incredible weakness, thanks to her absorbing all of the plague that was consuming Blighttown into herself. But, what if this wasn't the case, and in actuality all firekeepers need a constant stream of Humanity to live?
Quelaan gets it from her servants. Anastasia gets it from the enormous amount of activity (i.e. Kindlings and De-Hollowings) that Firelink Shrine sees. But where would Lady Darkling get her humanity? No one goes to Anor Londo—Undead have to make it all the way through Sen's Fortress, a feat that very few have ever accomplished. So, that bonfire recieves next to no activity, especially considering that there are three other, more conveniently placed bonfires for Undead to use once they are in Anor Londo.
In the "We are the Souls" video, the Lady Darkling is seen crumpled against a wall, as if severely weakened. Then, she's standing, being cradled by Lautrec. So, Lautrec is where she gets her Humanity. He brings it to her in Anor Londo in order to keep his beloved Lady Darkling alive. This is why he kills Anastasia; Firekeeper souls have lots of Humanity. Anastasia's in particular would have quite a bit, even for a Firekeeper soul, because of the aforementioned heavy activity that Firelink Shrine recieves. He did not kill her out of spite, or a self-preserving instinct born out of a fear of turning Hollow himself, but in order to preserve the life of his mistress.
This would also explain why when you can first encounter Lautrec - imprisoned in a cell in the Undead Church. He is found right next to another Firekeeper soul. Who imprisoned him is another question entirely, but it seems clear that that was the reason for his visit.
I'm sorry, are we talking about thePruld's "We are the Souls" video? You do realize it's just a fan video, as good and emotionally provocative as it is, nothing more, do you, so it can't be used as a source for theorizing game's lore to even a smallest extent