It’s been subtle but yes.
Here Sei is talking about how he’ll allow someone to die for his cause without a second thought, even if they don’t fully understand what they’re getting into.
Sei has also stated that his connection to the spiritual world is so strong, that he oftentimes has trouble distinguishing reality from fiction, which is a textbook sign of someone who’s sanity is questionable. Look at who Sei was before he was rescued from Zhao, that kid was legitimately psycho, and those feelings likely have not disappeared from Sei’s mind, but rather have just been buried down and will someday resurface.
The big tipping point for Sei will be when we meet Prince Entan of Yan, he’s the dude who will be mostly responsible for Sei’s madness coming back to the surface I think.
Sei will be the true final villain of Kingdom, no other character can fulfill this role, only Sei himself.