[CaptBlanc]

From what I read, and using judicious applications of Occam's Razor, I'd say that Madsen was acting from desperation.

Kim Wall gets hit on the head with the unsecured hatch. Pressure hatches are heavy. It cracks her skull and knocks her out and she falls down the ladder. Anyone who knows subs can tell you that a fall down a ladder can easily be lethal.

Madsen realises she's dead, and realises that he's facing a life-wrecking safety lawsuit. His boat, and his other upcoming projects, plus his finacial future and reputation are endangered.

(In the U.S., for example, he can be sued for the entirety of her projected professional earnings calculated from now till the estimated end of her natural working life. This can sometimes run into millions for some folks.)

He panicks. He decides to get rid of the evidence and makes up a hasty "Dropped her off at the dock." story.

Madsen attempts to dispose of Walls body, and scuttles the boat, assuming that no body nor other evidence will survive the sea. He's terribly wrong. He probably intended to have the boat recovered himself, so he scuttles her in shallow waters hoping the "bath" will clean it. He's caught flat when the authorities recover the boat first thing as a matter of procedure. Then the body washes up, too.

He's probably not a experienced killer. Just a man obsessed with his projects and in fear of his future. Faced with experienced police interrogators, he starts to come clean(ish). His inconsistant accounts of events indicate a man who's more interested in minimising any damage to himself than a bloke who want's to be truthful.

I'll be interested to see how this all plays out. It's one screwed-up tragic mess, for sure, either way.