This is so conflicting. I don't like that Dowun is coldly using his brother like that, but I agree that Do-Gyeol spoke too much and it's his fault another person was punished. On the other hand, Do-Gyeol is very young and was deceived by the other princess.
And yet, ultimately, it's the Emperor's fault: he's not a natural disaster or the embodiment of karma, but a man. It's because of him Hong Lyon was living in precarious conditions, he's the one who fosters a court where ass-kissing is the key to success, and, well, let's be honest, Do-Gyeol was right about basically everything he said about him.