6-5 had some brilliant subtleties though if you were paying attention
It's mostly about Inga:
Everything he plotted in the background was all to lead towards Rayfa being the unanimous leader of Khurain. Despite already knowing Rayfa's origins and the controversy that got Dhurke wrongfully booted off, he still loved Rayfa like his real daughter, as evidenced by his vault that was heavily guarded not having much outside a letter from Rayfa when she was little telling him how much she loves him as well as a paper that detailed a plan for dethroning Garan. And they also reveal later in the case that Inga's plans for a coup wouldn't really have benefited him whatsoever because a male without the royal blood couldn't lead anyways. You could also theorize that him killing Dhurke was out of jealousy or perhaps a reluctance to let Rayfa figure out that he's her real dad.
The thing is, they really gloss over it and let the readers figure out for themselves. It's a lot better to take in the case thinking Inga's a disney villain anyways. ---