I think cris is the killer because at the end of the last episode spoiler alert Ronnie dies (not sure if everybody saw that episode) at the end when they are eating dinner Giles says to cris "so you have never got a scared card hmm" cris nods Giles speaks again "how interesting" and the next episode when they don't know if Melina is dead or alive before that Giles is shown kidnapped and tied up with guns pointed at him and if you look up WhoDunnIt wiki if you scroll down a little you will find a bored that says the people the weeks and who has been murdered everybody who has been murdered is in a line and cris is on top as if she will be murdered last but you never know they could of just put it there by first letter of the name or they could change it up or just a little hint for us.
And that is why I think cris is the killer