@Lol king
Spoiler!
Warning: Major spoilers ahead!
Out of remorse for not being able to save his own son, he allowed Gu Yangcheon to kill him. It's difficult to explain, but as we can see, his father isn't particularly emotional. He knew he played a significant role in why Gu Yangcheon became the way he was. The bitter part of the story is that Gu Yangcheon sought out his father not to kill him, but to be killed by him. Otherwise, it would have been impossible for his father to be killed, except by the Heavenly Demon. The two had already fought each other, and his father even managed to seriously wound the Heavenly Demon. Unfortunately, he was also tempted by demonic energy in the process, even though it didn't seem to bother him much.
Out of remorse for not being able to save his own son, he allowed Gu Yangcheon to kill him. It's difficult to explain, but as we can see, his father isn't particularly emotional. He knew he played a significant role in why Gu Yangcheon became the way he was. The bitter part of the story is that Gu Yangcheon sought out his father not to kill him, but to be killed by him. Otherwise, it would have been impossible for his father to be killed, except by the Heavenly Demon. The two had already fought each other, and his father even managed to seriously wound the Heavenly Demon. Unfortunately, he was also tempted by demonic energy in the process, even though it didn't seem to bother him much.