The main reason behind this is because he lives for fighting rather than fighting for living. So, before going deep into this topic I would like to tell about Yujiro Hanma in a detailed manner to get to know about him more as he is our favourite character.
Our main character Yujiro Hanma is the possessor of the Demon Back which makes its user extremely powerful and strong as well as he is also the member of the Hanma bloodline.
Demon back is an extremely powerful ability which requires flexing of one's muscles to certain point where it began to form the shape of a demon giving it the name of Demon back.
After activating the Demon back the user becomes extremely powerful in every aspect whether it be physical capabilities, strength, endurance or stamina etc. and acheives inhuman abilities prior to the transformation.
It can also be said that it is a sort of test which can brings out exponential strength from the inside of a user. This sort of techniques which is used in this manga have been used in other manga too making this one no exception.
At this particular time our main character Yujiro is known to be the strongest man alive on the planet and is also given the nickname of Ogre. This nickname suits him very well due to Yujiro's appearance.
Coming back to the answer why Yujiro Hanma is so cruel. He seems to be following the philosophy of the survival of the strongest. According to Yujiro the person who begs for mercy is the the weakest. He doesn't believes in any kind of emotions like love and romance.
Showing anyone any sort of gratitude is a sign of weakness which should not be shown. Yujiro is a very well crafted antagonist who doesn't show any sort of kindness even to his own son.
Now let us find out where does his cruelty comes from. The most understandable answer for this is his father Yuichiro.
Yuichiro once his battle against Yujiro defeated Yujiro very badly and even if Yuichiro was not present there he technically won the match against Yujiro which leave a deep impact in Yujiro's heart.
After tasting the defeat Yujiro gets boiled up and in order to defeat his father he trained hard to surpass him. What we know about Yuichiro is that despite him defeating his son he don't allowed Yujiro to pick up on his grandson Baki. Which shows a polite side of Yuichiro which is not present in Yujiro.
We can also say that they are complete polar opposite of each other. We can know more about Yuichiro based on his facial expressions and features which are far less intense than that of Yujiro. He is also more calm than Yujiro.
After tasting defeat once was the phase where this anger of his derived him to be the monster that he is today. It made him a brutal bastard as we know him now. He developed the mentality that whoever who is stronger than him is just a stepping stone in his life which he needs to overcome in order to surpass his father and giving mercy to someone is just a waste of time according to him.
It can also be said that he developed a sort of inferiority complex whenever he saw his father's presence.
This thing motivates him in a right direction as he trained hard to become the strongest person on the planet ever for it he did whatever he could do to just step forward and to face defeat ever again in the life.
It is also kind of weakness at one point as the thing which he considers is his weakness is Baki's strength and vice verca can said. I said that it is pitiful because the same son whom he nurtures in order to be a worthy opponent for him came very closer in defeating him which he was not able to digest.
He never thought that someone who walks on the opposite path than him can come closer to defeat him but it was his own son Baki who came close. He felt defeated inside as he became to question himself that is he the strongest man on the planet or not?
So, these are all of the reasons which make Yujiro Hanma so cruel.
Which Characters Can Give Fight To Yujito?
• Musashi Miyamoto
He is known as the most powerful samurai ever and one of the most powerful person to live. He was able to give good challenge to Yujiro.
• Baki Hanma
He is the son of Yujiro and the half brother of Jack Hanma and is also protagonist of the manga. He is also known as Champion of the underground arena. During the fourth manga series he became the world's strongest boy. He is the one who came very close in defeating Yujiro.
• Yuichiro Hanma
He is the father of the Yujiro Hanma. He is the only person aside from Yujiro who was able to handle US soldiers on the small island. He is also known for defeating 2,000 US soldiers. It is a speculation that Yuichiro is the only person whom Yujiro can't defeat.
He is the primitive man who was trapped inside in a perfect rock formation. He is a giant humanoid who lived around Jurassic Era. He was able to kill dinosaurs without the help of any weapons and he also ate them. His teeth or canines are strong enough to tear any skin. He is the one who can make any big animal run in the sight of him.
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• Nomi No Sukune II
Nomi No Sukune is the descendant of the god of Sumo who is also named Nomi. He is a professional sumo wrestler. It is stated that his fist can even convert diamond into dust. He is also the only person who can defeat Yujiro.
So, this is the list of the person who are able to defeat Yujiro or have the chance to do so.