It obviously comes as no surprise that Raymond Clark was arrested. He actually has looked quite frightened since we first saw him in the back of a car when the search warrant was issued. Even now, he doesn't appear to look at anyone straight in the eye for any length of time.
As far as who the killer is, I really hope that the police have this right.
As far as motive, I don't believe for a second that IF Clark did it, that he killed Le over how rats were not being cared for. I'm quite sure that because he's worked there since 2004, she's not the ONLY one he has complained about. If he were so easily set off, he'd be a serial killer.
We may never learn the real motive. One thing is for certain, if I saw Raymond Clark on the street, he's the last one I would peg as a murderer. He definitely appears to have a mild manner about him.