I don't think it was Mercedes, she has just come out of hospital with a stab wound, it would be pretty damn hard for her to rush up the steps, get in, strangle Lindsay, lay her on the sofa, rush down the steps and then find Riley.
I think the reason it showed her acting shifty when she found out about Lindsays death was to point the suspicion at her so it'll be a MASSIVE shock when it comes out who the real killer is.
As much as I would like to think it is Silas I really don't think it is, it would be all over the national press that he had broke out of a high security unit and he would be suspect number one.
I don't think it was any of the suspects that they want us to think it is x x x