After killing the gargoyles on NG+ and having a chat with thechap in black tights who likes to laugh a lot for no real reason, I found I had been automatically cleared of all charges/sins from my previous play-through.
Did you join Way of the white for miracles from Petrus, or ask him to abandon covenant first?
By laughing a lot I'm just making a comment on his departure line 'It is only human to sin... EHAHAHAHAHA!. Or something like that.
I had no sin when I first spoke to him, and even made a point to check because I had a boat-load of souls from killing the final boss, and generally slaying things, and intended to clear my name. I asked him to abandon my covenant (Dragon, also under the pretense this would not incur a penalty) and then found out I'd been awarded sin for leaving the covenant.
I killed most NPCs pre-end game of my first run-through. Started NG+, blitzed through to Oswald without breaking my covenant, attacking NPC's or doing anything else that would incur penalty, beat down the gargoyles and found out I had no sin accrued.
It wasn't until after I asked him to abandon my covenant that I had been awarded sin again.