Now let's discuss about Kibutsuji muzan and tsugikuni yoriichi. They are the 2 strongest but opposing characters in the Demon Slayer/Kimetsu no Yaiba story.
Kibutsuji Muzan is the main antagonist who represents human nature at its worst which he is a selfish, narcissistic demon who has the principle of living only for himself without caring about how many parties suffer and are harmed by him. His obsession is the craziest side of humans with all their greed for eternal life, but being young, very strong, able to do whatever they want and being rich. In fact, he doesn't hesitate to kill anyone who gets in his way. Even though he still has a fatal weakness that continues to haunt him even though he has lived for more than 1000 years, a weakness that makes whatever he achieves feel meaningless if it disappears without a trace, including himself. He was a scary figure and hated by everyone.
Meanwhile, Tsugikuni Yoriichi is 180 degrees opposite to Muzan. He comes from a prominent family but has great empathy and was willing to give in for his brother, abandoning all ambitions to occupy an important position even though he was born a genius and tragically he also has to lose everyone he loves. Despite all that, he did not change but remained a selfless person, always helping those around him until the end of his life. He'd never prioritized wealth, power, strength but only wanted to live in peace. Even though being a good person is not always rewarded in the end, he didn't change. Thus making him a legendary and memorable figure. He was also the only figure who made Muzan felt insecure because he would almost perished if he didn't escape quickly like a true coward.
So what lesson can we take from here? Always be a good person no matter what happens because karma never makes a mistake and this world is not eternal and life has an end. Don't follow your desires which will lead you to negative things.
