There's so little story communicated to the player, that I didn't realise a knight I had saved from a cell in the Cathedral (near the first bell) ended up killing the Firekeeper at Firelink Shrine.
Anyway, a black orb was vibrating near the bosses in Anor Londo, so I used it and 'invaded' the knight and managed to kill him on the second occasion. He dropped a fire keeper soul and I didn't think anything of it so upgraded my Estus flask at the nearby NPC in Anor Londo.
Turns out that was the girls soul and I could have bought her back to life! Whoops.
I also killed Solaire of Astora early in the game and it looks like he would have been available to help with lots of boss fights, so I've effectively made the game even harder.