So I accidentally killed Orniex in the shaded woods. I'm a sorceror so having a look at the things I can buy from her, it doesn't look like I've completely screwed up.
How "game breaking" would my mistake be? Am I still able to finish the game without too much trouble?
Yes, you still can end the game without that npc, she is optional and her only function is to craft spells or weapons for boss souls, so for this playthrough you are locked out for this option but it won't hinder you to finish the game.
The other thing that locks you out is in getting some achievements but that is a different question.
Ornifex is completely optional and has no other impact in the game than giving you the chance to exchange boss souls for spells and weapons, buying some weapons as the Murakumo and consumables and, of course, a bit of her lore.
If you have killed her expect to find her tombstone after some hours of gameplay, you will still be able to trade the souls and talk to her for some spare souls to resurrect her.
