I really believe that the reason the movie "sort of" makes it open ended is because otherwise Robert Durst wouldn't have given the movie makers the time of day. He probably wouldn't have answered any questions or done the commentary.
However, I do think the movie is trying to say that he did it. Even though it's supposed to be "open ended" it's pretty clear cut to anyone with half a brain.
Good article though. It really sickens me how lousy police work can be. I really think that's mostly to blame here. There was probably craploads of evidence that just got thrown away or destroyed.
I mean, give me a break. No way was Robert some kind of suave killer that was able to hide his tracks. Just looks at the way he killed and hid the old man. It was sloppy. It was most likely the same way with the other killings but maybe he got away with it due to it being in another state.