The East Palace k-drama ending

I just finished watching The East Palace ( kdrama) and I'm so confused about the ending. Lol how did gu cheon come back to life?? didn't he become an akgwi ?? I'm so dumb i can't understand anything 😭 please someone explain me. Lol.. i have few more questions like why did the king kill the crown prince when he was just as bad as him ? He could've joined hand with him lol because he did the same thing. It's not like the crown prince was going to expose him he just did what his father did. It was so stupid the king killed him i mean i would've understood if the crown prince was good and the king feared he would blow his secret 😭 anyways I want to understand the ending though.