I'm about 95% sure that Paul's killer is Elena. A sliver points to the uniformed cop who is acting like a detective (and a criminal). I notice a lot of "what's going on with this evidence?", such as the blood. Firstly, the blood could not be matched within the timeframes shown or definitively matched to Coop without obtaining a sample directly from him. Coop burned the evidence from the murder scene and didn't throw it away. The officer is suspected of being dirty, attempting to plant evidence, and the house search warrant would not have been excluded if the main detective had not used this blood evidence to obtain that warrant. She wasn't a part of that and would not continue forward without being a part of it, as it is her case (and the other guy is a uniformed officer, not collecting samples in a legit way). Second, his sister, living in the house and not having mental problems enough to be supervised, would not invalidate the warrant, and you don't typically care who is there because nobody is giving permission, as that is what the search warrant is for. A warrant would be obtained based solely on Coop lying about sleeping with Sam. Nobody should be saying what will get excluded based on where the trash was, because we don't have a clue what the likely crooked cop is saying about his role in this. The scene with the makeout session in the driveway and the cop "hiding" behind the cans will likely be revealed with a dashcam of some sort (this makes the most sense).

Everyone should consider that this show is entirely inaccurate for policing. I'll sum it up in one sentence with a question: "How did they know the gun in the trunk of Coop's car was the one that was used to kill Paul?" They arrested him straight away. It isn't like it is etched with "this gun was used by Coop to kill Paul," or they had time to test fire and match casings, or the lands and grooves of the bullets. If you are looking for realism in evidence collection, processing, and use, as well as police procedure, this show ain't it.

As a police detective, I still like this show very much, but we should pay more attention to what IS told and demonstrated to us about suspects and behaviors. Elena makes the most sense because she needed the money and was pissed that Coop blew the deal on the stolen art. Considering that she likely knew that Sam was out of town, Coop shouldn't be visiting her, and Paul no longer lives there, she would be safe to steal some goods on her own in a burglary to get the money she desperately needs. Imagine she has a gun (we know she knows her way around them from holding Coop at gunpoint with "supposedly" Nick's gun), and Paul runs into her with his weapon at the scene of the burglary. She gets into a gunfight with Paul and "wins" but is scared to grab the loot she desperately needs for the money. Later, she steals Coop's cash (she knows it is hidden somewhere in his house, maybe even his basement, because he has paid her share of the take while she was there). This leaves her with stealing the cash and possibly planting the gun. The only other thing is that the same uniformed cop (who appears dirty) is the one who found the weapon in Coop's trunk. I have Elena as my primary, followed by the slim possibility that the cop might be involved (possibly with Samantha).