Its more of a personal portrait than a thriller. A depressing portrait. The disastrous police investigation was mostly shown in the first parts of the series.
The hypothesis is not bad and Engström is clearly someone the police should have investigated further - but ignored. However...the explanations - the constant insinuations the creators have given Engström to enigmatically mumble from time to time - feels somewhat forced a lot of times. Every one of Engströms odd behaviors that is presented to the viewer and that tells you why he had the motivation and did the murderer, can easily just explain that he did and said all the contradictions just because he wanted some attention. Add the alcohol problems and the results will probably be confusing. It could have gone either way and we will never know.
To sum up - decent theory but not convincing.
The hypothesis is not bad and Engström is clearly someone the police should have investigated further - but ignored. However...the explanations - the constant insinuations the creators have given Engström to enigmatically mumble from time to time - feels somewhat forced a lot of times. Every one of Engströms odd behaviors that is presented to the viewer and that tells you why he had the motivation and did the murderer, can easily just explain that he did and said all the contradictions just because he wanted some attention. Add the alcohol problems and the results will probably be confusing. It could have gone either way and we will never know.
To sum up - decent theory but not convincing.