I liked the way it ended. It's a bit of a cliche that the killer isn't revealed until the last moment due to some clever detective work. With Broadchurch we did have detective work, but much like most real life cases, it was something else that revealed who the killer was. Having him turn himself in also gave plenty of time for the reason to be explained, although you were still left wondering how far he would have gone.
It also gave more time for us to consider the impact of the crime on everyone in the community not just the murdered boy's family. This was a major theme throughout the series.
Finally the ending had an especially big emotional impact because of who the killer was.