I was actually wandering what would be necessary for Rowan to get that same treatment
I think he would do it if there was something significant he would gain from it, or possibly if he was betrayed. Killing Rowan would only bring the twin’s wrath down on him as things stand now so he would have to be really pissed. Maybe if Rowan suddenly holds all the power by defeating the twins and then goes on to deny him the land he was promised he would do it, or if he sides with Che-Lin or Zii-Zii and completely fucks up his plans to become king and unite the goblins. Even with his fanatical love for Rowan I believe it’s still possible, and his betrayal would only cut deeper because of it.
If he truly loves Rowan he should gracefully step aside and let him be the goblin king, since he bases all his strategies and tactics on Rowan’s exploits anyway why not let the original take charge? Then Rowan can marry Che-Lin as queen and live happily ever after. Or he could marry Zii-Zii, or even say fuck it and marry them both plus Tue-Mai, Cla-Min and all her female family members. Tue-San can be sent to the dungeon to be sexually dominated for the rest of her days because that’s probably what a control freak dom like her would hate the most.
The goblin harem end we all deserve, this is obviously the real reason the goblin arc is taking so long, Rein is patiently masterminding the true end for us
Edit: Perhaps it would be more degrading for her to join the harem and be paraded around as a submissive wife, fuck it you can also join the love Tue-San.