I don't care at all for clip shows... but this one was fun. I even enjoyed the plot created to tie together all of the clips. And yes... it's been our impression that the killer is just what LaxScrutiny
said: the killer is the first (or nearly so) person we so with no apparent motive for the crime.
Even with its lapses (such as the aforementioned seeming Castle buffoonery) I still enjoy this show very much.