Genghis Khan suffered a lot when he was a kid. After his dad died from the enemy tribe’s poison, his mother and siblings were driven out of their tribe and left to fend for themselves. Genghis Khan himself as a young kid was once captured by the enemy tribe and and made to carry a wooden beam on his back for a while, and he had to run away from the enemy with the wooden beam on his back.
He being a very vengeful person from young, even killed his half-brother over a minor issue.
But I think that he could attribute his success later in life to the strong determination to ensure that never again would he be bullied and taking advantage of again. That was why he became ruthless and cold-blooded as he believed that was the way for him to achieve his aims - understandable considering the type of environment he operated in.
Also, he had to avenge his father’s death and his father being the leader of a tribe, Genghis Khan felt that being at the top was his birthright - it just happened that his father’s enemies had deprived him of the chance to be the leader.
Plus he was naturally a smart guy too. He had the leadership qualities of his father from the start.