I enjoyed the series. Part of me thought it was a bit of a cop out with him killing himself, especially as they can't have him in another series, but I think it actually made the ending very poignant.
For me it was to do with, not just him, but the fall of others around him and how many people were impacted by him, some significantly so, his family for one.
I thought they were right to carry on with the story about Katie and the effect it has had on her. Similarly Rose. Even the Defence solicitor quitting her job.
I think the senior policeman was a bit of a drunken mess anyway and the incident in the police interview pushed him past the point of no return, especially as he seemed to have feelings for the one Gillian Anderson played.
Unless I missed it, it still wasn't clear if the memory loss was real or not, for the later murders, especially given he could remember the details of the earlier London killing.
It was all building to get him to face justice and punishment and then after everything that happened and the affect it had on so many people, he killed himself and no justice can really be done.