I killed Petrus, what do I do now?

The reason that the other NPCs are attacking you is because you killed "another member of their order".

Normally, going to Oswald to "absolve your sins" should clear the slate, but I'm not sure why or if this should have worked.

Petrus has his own story line with the other three NPCs. Once you collect the Lordvessel, they will venture into the Tomb of the Giants, and get trapped. You can find them, kill the two guards (as they have gone hollow), then Reah will return to the Undead Chapel.

She will stay there for a short time (2 or 3 bosses), so you can buy miracles from her then. After that Petrus will kill her.

Source

The issue is that you unfortunately killed Petrus too early, so you have unfortunately ruined the story line for these characters. Ideally, you should kill Petrus after you rescue Reah from the Tomb of the Giants.

Unfortunately, I believe that since you killed Petrus before they arrived, they will be permanently aggro'd. The only way to resolve this is start the game over or enter ng+; or kill them.

You can make this NPC friendly again - you need to request an Absolution from Oswald of Carim.

Or you can just continue your game as is. Some Miracles are indeed only available from this NPC, but there are many others you can find somewhere else, see Miracles. And if you want to collect all of them, you can always go to New Game Plus and complete your collection there.