I don't know how important Petrus is but he engaged me after I attacked him and I had to kill him because he started attacking me on sight. After I opened The Depths, I went to talk to that priest that's hanging around him, and she attacked me, too, along with 2 guys. I wanted to buy the heal spell from her. Can I still finish the game, or even make them neutral? Also, I killed the vendor right after the Gargoyle fight under the bell tower...

.......... no

You cant make them neutral (until you get to a new game +) and yes you can finish the game , you just need to find the cure curse drops , but I dont remember were its been a while since last I played Dark Souls

You can finish the game irregardless of who you kill, infact when at the last boss and about to finish its recommended you do kill every npc. You basically loose whatever service or help they would provide if you kill them.

Is there another way to get the heal spell?

There is a cleric blacksmith at the start of old Londo ruins, before you even get to the ghosts, but I dnow of he sells heal.

You can still get curses dispelled by Ingward in New Londo.

You killed two miracle vendors and the NPC in the first Bell Tower is Oswald the pardoner.

Basically you won't be able to buy a bunch of miracles or absolve your sins, had you not killed Oswald you may have been able to reset any angry NPCs but that is no longer an option for you. I'd try not to piss anyone else off if you value their services.

And yeah, you can still finish the game. No NPC is really that important.

@LordXavierBritish: Thanks dude. I always accidentally attack people when I tab out to look up who they are, I tab back in and misclick to crack them in the head, and it's game on.

patches sells some .. i thought he sold heal but this wiki only says he sells great heal excerpt .. which is odd

@envane: He mostly picks off stuff from clerics that he despises for an odd reason.

@Nightly1029: Well for your first run, you can think of it this way, It's completely different game now that you have lost their service which will probably increase your "Instincts" regarding your ability to survive as many encounters that you come across.

Irregardless is not a word

@Tim_the_Corsair said:

Irregardless is not a word