@Aniryu Once again I am late, but...
I think at this point they didn't have a choice. Trying to force the MC into marriage as a concubine, not even a wife proper, was like a slap to the face at best. When they didn't meekly accept their daughter being shipped off like a whore or cattle he tried to assassinate the head of the family. A man that, as far as we have seen, has been nothing but loyal and supportive to the royal family. At that point I don't think there was a way to end it peacefully. He basically declared war on their family. Even then they didn't retaliate, they abided by their family's word/code/honor/contract and gave up their nobility/standing. That was the biggest olive branch they could really extend, it was them stating that they didn't want to fight. If the Emperor was sane he would have just dropped it and forgotten about them. Instead he pushed again/farther, at that was one step to many. Only other way they could maybe have settled it peacefully would be if the whole family/clan packed up and moved to another country, which isn't a simple or easy thing to do in their era/time. Even then there is a good chance the Emperor would still send assassins after them to make sure they didn't "turn" on him or become a problem. The 3rd Prince saw it all coming, he even tried to warn his father but he wouldn't listen. When the prince left after their last fight I got the impression he knew that his father was going to die for his idiocy.
Saddest thing is that if the Emperor had just patted the family on the back or ignored the lightning he would still likely have the solid support/loyalty of the Fengs. Frankly I think the Emperor was either legit going insane or was being manipulated/driven insane. Some of the leaps of logic and actions he took didn't seem like the actions of a sane person.