I just finished watching this on PBS, which has been showing it as a repeat. I was kind of disappointed with the direction it took in part three, when it chose to examine heavy themes of war and racism rather than provide an involving mystery and the appropriate clues. The lack of such nuts and bolts of a good mystery gives the show the tone of a bland TV drama on commercial television. The build-up of the previous episodes regarding the theme of original sin and sins of the fathers was promising, with the stuffed snake toy suggesting intriguing possibilities. But the third part takes this theme to a melodramatic letdown of an explanation.
But what I'm most disappointed with is the absence of a climactic revelation, in which the detective pieces together how things really happened and explains it to other characters (and thus to the audience as well). In mystery shows of this sort, flashback is often used, and it's fun to watch and see what took place and how. Not only did this episode dispense with flashbacks, but also with any sort of explanation, period. Not even a denouement to wrap up loose ends. Apparently the filmmakers didn't want to bother, and instead thought that the histrionics involving the killer's suicide would leave viewers satisfied.
A fatal decision, since they left behind a giant plot hole --- the killer actually had an alibi for the second murder! The person who turns out to be the killer tells the police that he/she was at a public institution at the time the victim was being strangled and dumped in the river. This story is then verified for the police by other nameless witnesses off screen, a common device in murder stories meant to erase any doubt about the character whatsoever. But there is no attempt made to account for this alibi, no revelation that a witness was lying, mistaken, etc. We are left confused, with the knowledge that the character had an alibi and couldn't be the culprit, yet did indeed commit the crime somehow. Very sloppy on the part of the filmmakers.