the other duke is def khalid but why is he a knight in oshen(?) when he's a duke and the "...

This all just my assumption. Khalid might be a traitor in the first place. That's why he killed MC(Ishtar)?

Or he might just be a psycho who killed the girl he loves to make sure no one get her. ╮(￣▽￣)╭

Some men's love is precious, and they would die for their love. Other's love is just cheap, and they would sell it for profit.

Spoiler
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Khalid did that cz it was necessary thing to do. I will not spoil more

Then he should have made it a murder suicide. Like I kind of understand why he did it some people just don't understand fighting till everything is destroyed to protect the people? Sometimes wars just need to end buuuuut still 100% never forgive betrayal he shouldn't be living it up as a noble instead of just being a guilt-ridden mess.

Spoilers what i heard on FB
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So fl was working for the king maybe? Apparently that king is not all good fl thought on previous life.she even put her life on stake for the empire but empire is on the bad side which our fl didn't know.But Khalid knew that so he killed the fl so that People wouldn't point fingers on her and if he didn't do that our fl would considered as villain. And she would be heart broken cz she did everything for empire not knowing that their king was the real (villain)?
Im really sorry i suck at explaining things -_-