It's like he wanted the brother to react to his lie.  He wanted the old brother who would tell him It's wrong to lie, but that brother died the moment he stepped inside the snow prince's House. It's like the brother now view's the world more based on numbers and threats and to him.  Sehan's lies is just child play but to Sehan that is like a gut wrenching guilt because he was taught to be more honest by the same hyung whom thinks lying is the Same as being kept from.