Why is Quelaag hostile?
There are several valid guesses as of why Quelaag is hostile from the get-go:
- She protects her sister from any intruders, regardless of motive, to provide humanity for her sister.
- Considering how long she's been there, Quelaag probably doesn't bother questioning motives of intruders at this point due to how hostile literally everything else is in Lordran.
Quelaag's sister - the Fair lady - is blind and weak. Spoiler:
if you have the Old Witch's Ring, you will be able to understand what she says, and how she mistakes you - the chosen undead (who can feed her humanity to ease the pain and open a shortcut) - for her caring sister.
Lore-wise, the Fair Lady was transformed into the spider thingy by the Flame of Chaos incident. The reason to why she is weakened (and thus protected by her sister) is because she's attempting to cure Blighttown by sucking up all the blight (Eingyi being one of those whom she helped).
She and Quelaag were two of the only three (the third one being the witch Quelaana who managed to escape with no mutations) whose sanity survived the Flame of Chaos incident that mutated everyone else into demons (as their torsos are still human, so are their minds).
And you killed her sister to ring a bell, and now she's all alone with nobody to care for her, you monster.
For further info and clarification, refer to articles on the Fair Lady, Quelaag, and Quelaana on their wikidot pages.
There are actually unused lines of dialogue in the game relating to Quelaag which imply that she may have been planned to be an optional boss at some point during development, much like Priscilla.
The following lines exist as sound files in the game data, and you can listen to them here.
Go back. Forbidden be, these parts. The realm of the creatures of chaos. They accept their banished fate.
Go back. Lest the flames devour all, and the children of chaos feed upon your charred ashes. Those who defy the pact… Those who trespass Quelaag's domain… May you feel the depth of our wrath!
Ahh, a precious new sacrifice! Forbidden be, these parts. The realm of the creatures of chaos.
Go on, go on ahead…
Welcome, bringer of meat. The children of chaos are hungry; give yourself to Quelaag's flame!
Keep in mind that speaker of these lines can't be confirmed as the lines only exist as sound files, but they seem to show that more of a story was planned for Quelaag and the chaos covenant, which didn't make it into the game for unknown reasons.
It's would make sense if Quelaag being a non-hostile NPC was cut due to development constraints, as much of the areas between Quelaag and the Bed of Chaos, including the Bed of Chaos itself, show signs of being unfinished compared to other parts of the game.
Quelagg is hostile because she is protecting her sister, the Fair Lady. You are an intruder, so she attacks you, regardless of what your intentions actually are, because she can't take the risk that you might find her sister and kill her.
Furthermore, you are obviously not hollow (as in, insane mindless zombie, not hollowed as in offline mode), so if Quelagg kills you, she can (hopefully) get some humanity from you to give to her sister to ease her pain.