By 1206 he had unified rival clans and taken the title Genghis Khan (or Chinggis Khan), meaning "universal ruler." Over the next three decades he led a constant military campaign that ravaged vast areas and subjugated millions of people, earning him a reputation in the history books as a brutal monster.
He adapted his method of warfare, moving from depending solely on cavalry to using sieges, catapults, ladders, and other equipment and techniques suitable for the capture and destruction of cities.
He is infamous for slaughtering the entire populations of cities and destroying fields and irrigation systems but admired for his military brilliance and ability to learn.

At the time of his death (1227), he had conquered the territory from China to the Caspian Sea.
Many people, historians and laymen alike, have regarded the Mongols of Chingis Khan primitive and terrible barbarians, and satisfied themselves with such a one-sided, superficial analysis.
Why not instead try to ask how it is possible for humans to achieve so much? Theirs was an upward trajectory, a series of accomplishment so absolute, so unparalleled, that every historian trying to delve seriously into the matter will find himself amazed, if not awestruck.
The Mongols had evidently broken out of the normal adaptive limits of humans. They had to do so, in order to accomplish what they did.

He once killed his little brother from an argue over catched fish when he was a teenager. He really had a harsh childhood life since his father's death when Genghis was 9 yrs old. This made him a tough man. The reason people consider him brutal is the slaughters he made during military campaign. After defeating Tatar, a rival clan, he ordered execution of all Tatarians taller than wheel of an ox cart.
Conquering countries doesnt mean he hated them. He just wanted to unite them under one rule.