SPOILER AHEAD: Serena did not 'technically' kill someone. A guy named Pete was kissing her (etc). This was right after she slept with Nate. She felt so terrible about betraying Blair that she wasn't into it and handed him the drugs he had brought and said something along the lines of, "i'm not in the mood,". He apparently overdosed and Georgina dragged Serena out of the room so that they wouldn't get caught.