I thought that Sophie's boyfriend was the one creating all of the trouble because he had an inappropriate crush on Lindsey. I was way wrong.
Marcus, the stable one of Lindsey's friends, was actually the husband of the woman that Andrew killed in the accident eleven years ago. He was out for revenge. He wanted to hurt Andrew and thought that he could do it by framing Andrew for stalking Lindsey and then kill Lindsey and Sophie, also framing Andrew.