Hi, I (German, not the guy with the submarine) do not speak Swedish so I post in English here. I am also interested in this case and therefore read with interest in this Forum (using the google translator 😊

and also write in a German forum. I hope you do not mind if I post my opinion here.
I do not think Mr. Madsen planned to kill Ms. Wall during the trip. Everything he did seems “improvised” to me:
1. The hard disk: I would never leave an unencrypted hard disk with my “treasures” in my workshop where anybody can find it.
2. The saw (Fuchsschwanz): if I plan to dismember the victim I never do carry it like a monstrance in front of me into the boat so that everybody can see it.
3. The blood: If I plan to dismember a body I put it into a big plastic bag or put something under it before I start ...
4. The pipes: If I plan to dump a body (body parts) I take some dumbbells or other compact heavy weights to the boat which I later put together with the body parts into a duffel bag or something similar I can cord up tight. I do not use lightweight pipes and try to string them on a rope.
5. The immersion of the boat: I do not dump the boat in shallow water not far from the coast knowing that all military primary radars can see me, data is recorded so one can reconstruct the whole route of the submarine exactly later on with all the times when it dived and came up again.
6. Some of Ms. Walls clothes: I would not forget clothes of the victim on the boat, as I must take into account that everybody wants to know if she could be still in the boat after being missed.
7. His lies: instead of having prepared a partly reasonable story like:
“After diving I went out of the boat with my remote control and started the diesel. I expected M;s. Wall to come out after me. Suddenly I heard a loud clap and saw that the hatch fall closed. I tried to stop the engine with my remote control but realised that it did not work. Then I saw that the cable was cut off. So I tried to open the hatch but it was not possible as the engine evacuated the air from inside so the under-inflation was too high. I tried and tried but is was not possible. After a while the engine stopped. After some minutes I managed to open the hatch. I found Ms. Wall dead on the ground under the hatch. Her pulse was imperceptible, she was dead. I became angry, something came over me. I can’t remember what I did. Later I realized what happened, I dismembered the body and used what I found on board to dump the parts.”
May be he would not come through with such a story, but her could cut off the remote control cable with the hatch to make it look like it. And if the body was not found or the cause of death cannot be determined anymore (as it looks like now) and the story with the under-inflation can be reproduced he can have good luck.
I personally think, something triggered his anger and he killed Ms. Wall in the heat of the moment, followed by the necrophilia.
But of course any other scenario is possible as well as, in my opinion, there is no enough evidence for a clear progression of events.