Unless they've not shown us the full bloodstain there is definitely not enough for a serious head injury, theres no evidence it IS laurens blood either!
They have no body, they don't seem to have spoken to Evelyn at all who would coroborate Roys version of events and yes, he didn't attempt to clean up all the blood, which he would have if he were the killer!
They should also be aware of Laurens connections and what nasty pieces of work they are, and yet think Roy is a more likely fit for murder than any of them? Bizarre.
But then these are the police who failed to get into Stephens laptop files when Daisy, the reknowned tech-genius l33t h4x0r managed to do it in mere minutes!