what i think is that the elder brother harbors a DEEP grudge against chang-geon for a) usurping him from his rightful place as emperor, b) killing off his entire family (regardless of the reasons why), c) for taking away his trusted confidant/friend/peer (idk where sam myeong stands with this guy tbh); so he built a network of informants and supporters, and egged sam-myeong to get closer to chang-geon while making chang-geon anxious about this guy's interference, which would lead to him ending juhye's life (chang-geon only changed his mind in the last minute bc of how much he could emathize with juhye despite not wanting to) and now that all of crown prince's cards are arranged just right, he manipulated chang-geon into admitting that he killed sam myeong's family and hid juhye away on purpose, and also started instigating the people in the court and probably in the country to question chang-geon's legitimacy, and basically inciting a civil war. Let's see what s3 has in store for these people
dont see any coming back from this... im also really confused as to what the brother is trying to achieve.