He would be Joe, Ellie's husband, and that provides the major obstacle to believability. As Ellie herself says near the end of the episode, how could she, an excellent detective (my evaluation, not hers, but clearly true) have missed this? How could she have been "lying next to the murderer" she was looking for?
I don't see how, either. The similar looks of Joe and Nigel, a nice touch to explain Susan's incorrect, heartfelt identification last week of Nigel as the man she saw on the beach and therefore the killer, still doesn't explain how Ellie could have been so blind to Joe. Even the old adage that we can be blind to things too close to us doesn't quite explain this. Was Joe such a great actor that he not only fooled Ellie and Tommy, but everyone else in town? No one saw even a slight hint of his attraction to boys, a glance in the market or on the street?
But the finale was brilliant nonetheless because of the way it showed the impact of the revelation of the murderer on the town, especially on Danny's parents. Danny's father shouting at the sea, after holding it in for so long, was a major scene, and perfect for the deepest emotions that the sea can symbolize and set loose. Danny's mother not believing how Ellie could have missed her husband as the murderer was right, too, for the reasons indicated above.
And the last scene between Ellie and Alec - following several superb earlier scenes - couldn't have been better, either. Both detectives not seeing much of a future for them in the police force of Broadchurch - Ellie because of Joe, Alec because of his health. Ellie again comes up with a great phrase, saying they're a "former detectives club".
But there is a Broadchurch second season in the works, so we may well see these two again. Or maybe not, if the common thread of Broadchurch is the town and its events, with murder just the venue for the first season. Whichever way it goes, I'll be watching. The Brits know how to make television.
See also Broadchurch: Powerful Viewing ... Broadchurch 1.2: Brooding Excellence ... Broadchurch 1.3: The Spy ... Broadchurch 1.4: The Unusual Suspects ... Broadchurch 1.5: Good Loving and Almost Loving ...Broadchurch 1.6: "A Break from Being Sad" ... Broadchurch 1.7: Missing Links