How the hell is he going to kill her but explain it was because of sex? Cut off her head and say he wanted to throat fuck her but her skull was in the way? Bro's about to pull a weimar goose schtupping.
He reveals his identity to Liliana and with her assent seals her memories and gives her a fake personality of his dog. He then tells the night elves that she is his pet now and takes charge of her.
He tries to undo this after returning home but Liliana's brain keeps her memories sealed due to psychological trauma, so for a long time she walks on her stumps as Jilbagias' dog for comic relief.
Eventually when Alex has a massive battle against the 3rd prince(the one who destroyed his village in his first life) Liliana regains her memories and regenerates her limbs. Afterwards Alex helps her escape back to the elves' forest.
Eventually they are reunited afrer Alex/Jilbagias is exiled to human lands for one year as punishment for killing his brother, the 3rd prince.
Honestly not much of a taboo, when she'd pretty much be saving her.
The real taboo here, excluding him actually torturing her himself, would be to let her live in this state.
As a Warhammer40k fan, I'm getting really strong Dark Eldar vibes from the Night Elves (and the way the Saintess was tortured by them). That the MC is basically an equivalent of a champion of Slaanesh is also notable in this respect.