**spoiler**
He was following the orders of his father's demonic sect. The redhead is more like a pawn following orders, he's not the main villain or mastermind, he wanted to prove himself so his dad would accept him, that was the plan from the beginning, he always was a spy.
*Spoiler of main villain*
The main villain is the Count of the neighbor territory (the one with the strange fog). Basically, he always felt inferior to the Duke's late father, so for revenge, the Count wants to destabilize the south and replace the Duke (even though neither of the Dukes actually harmed him, the worst thing they did was to ignore him lol).
It's not that he's unimportant but he's more like a puppet. If you want more spoilers...
Even though he's an antagonist he isn't fully on board with the orders he's given, after realizing his love for the Duke he decides to not follow the orders, betray the evil guys and somehow protect the Duke. In fact, he saves the Duke a couple times.
I gave up and read the novel, fortunately the main story is finished. It was good and very similar to the manhwa! It was worth it!
The end of the 1st season of the manhwa is around ch 65 of the novel, if someone is curious. You can look it up as Crcumstances of the Fallen Lord.