I felt mixed about the ending.
On the one hand I was annoyed Spector didn't have to take any responsibility for what he had done in the long miserable term. Yes, the moment of dying probably woudn't have been that enjoyable for him, but he still got to die on his own terms, when he wanted to, and did so because he didn't want to endure being couped up in the mental place at best, or prison at worst. He got more choice of the end of his life than his victims, and oh yeah, got the enjoyment of hurting some people in the process. Stella said when interviewing him that death and avoiding system justice would have been too easy and he certainly understood that. His mind must have been a mightmare to live in anyway when free, so he prob couldn't foresee living behind locked doors for years not being able to at least prowl for a victim to get his releases.
On the other hand, while I'd have def wanted to see Paul convicted and the official conclusion come to that he was indeed a murdering lying scumbag, I think the ending prob put forward the message that there are plenty of unhappy endings where those that should pay for their crimes, don't always, and plenty never get justice and the misery of life continues. I think Stella looked upset when she saw Paul's body, not because he had died, but because it went right back to what she said about death being too easy for him and all that work, pain (mental and eventually physically) felt like it hadn't been wholey worth it. He was dead and relieved of responsibility, while a destroyed wife, fatherless daughter/son and dead/living victims all paying some price of unfortuinately coming into to his life. I wanted to see Stella and team finally get their man (after all his pathetic games), but this miserable ending for all in way felt like a very plausible reality to show too. We couldn't even feel sorry for Paul being abused as a child, because he never allowed us anyway, and due to his way of dealing with it as just not something to look beyond despite his childhood the cause/stirring of his ways. He was sick and wanted to stay sick, whereas as other sickos (of abuse) have tried at least to get better. He pref attention and the pain of others, rather than trying to be happy with others.
I did like the hug with Dani and Stella at the end (Stella couldn't really bring herself to return it, but I'm sure she needed it). Their relationship felt one of the most genuine, as Stella didn't need any sexual/romantic element there to benifit some urge/use as momentary unmeaningful distraction. She just trusted Dani and liked her as a person and I think cared for her without needing to say so or overtly show.
As she's back in England, does this mean we won't get another series. I want to see Stella again, now that we've finally been released from Paul, but they won't want to move away from the Irish angle of the show, so a new series could only happen if she was sent back to N.Ireland for another case. It would look bad though if the police team there had to always have a english bobby come and help them close a case, so surely after three series they'd remain Londonside if there was a new series. I wouldn't mind a Londside Stella, but can see they'd not want to turn divert from an Ireland location.