I think pretending as his brother is already possessive enough cause he wants to keep an eye on Taehyuk. They were childhood friends thus the fondness he has for Taehyuk and the obsession of keeping him all locked up for himself. This bit got the story more interesting. Just basing from the raws and the little korean I know (●'◡'●)ﾉ
But why does the emperor have to do all that crazy stuff? Like have Taehyuk all to himself? I mean, as emperor he has to be too busy to look at him all day and too busy even for his queen -_- And all he will see is Taehyuk looking sad anyway. And Taehyuk can't have cute babies the emperor can watch without getting to even spend time with his wife. It is just nonsensical to me.
What does the bro emperor want? Seriously. Obsessed how/what? It doesn't really say it. He gets rid of the friends, marriage aspects, etc. So....he can stare all day at Taehyuk...or be his only source of care/support? Like father figure? He's the damn emperor and has a whole nation depending on him already. So? He hasn't done any naughty touching, so I don't think he wants anything like that.
In present time he doesn't seem so super obsessive at all. Like he is curious, but not keeping tabs, GPS....