Peter Madsen, 47, claims Kim Wall, 30, died after inhaling toxic fumes on board his 50ft warship.
Cops in Denmark charged him with murder after he admitted dismembering her body and dumping it at sea.
He told a court in Copenhagen that he thought of 1995 hit Se7en as he cut off Wall's head – adding that he didn't think there was anything "unusual" about that.
The thriller includes a scene where Brad Pitt's character opens a box containing the severed head of his wife.
He said: "I don’t think that there’s anything unnatural in that remark.
“In that film, there is a scene where a person’s head is cut off.”
Madsen initially said he'd dropped Ms Wall off at 10.30pm at the Halvandet restaurant close to where they had set off, near the Copenhagen submarine dock.
But in court he maintained that Wall had died from carbon monoxide poisoning after they met on the submarine at around 7pm on Thursday, August 10.
He said they were trapped inside after Madsen started the engine with the hatch closed, and that Wall died inside while he was outside the submarine.
Last week the court heard how backup data recovered from Madsen's phone revealed he Googled "beheading" and watched videos of a woman having her throat slit before Kim's death.
Her headless and limbless torso was spotted by a member of the public in Danish waters near Copenhagen on August 21.
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