Genghis Khan had a lot of blood on his hands to be sure, but that is but saying in other words that he was about as violent as most of the other rulers in history. I think Genghis does not get nearly enough credit for the instances when he offered mercy to his foes or tried to negotiate with them. Here are two incidents which I think are particularly noteworthy:
1) In 1206, as Genghis Khan was completing the unification of the Mongol tribes, his last rival was his childhood friend, Jamuka. After Genghis defeated him, he offered to be his ally again, but Jamuka said that just as there there could only be one sun in the sky, there could be only one ruler of the Mongols. Jamuka did not want to live with the shame of defeat and asked Genghis to have him be executed. Genghis granted his request.
2) Khwarezm was a Muslim empire which occupied much of what is now Iran. Tired of war, Genghis decided to send a trade caravan to establish a trade route. The caravan was ambushed and many of its party were killed by a govenor of Khwarezm, who believed it to be a covert attack. Genghis later sent another delegation, this time directly to the Shah of Khwarezm. The Sultan beheaded the Mongol emissaries and sent back the heads to Genghis. Genghis ordered an invasion of Khwarezm, killed the Shah, and obliterated the capital, Samarkand.
Genghis Khan, I think, did not want to spend his whole life at war, but simply recognized that violence, if used properly, can be extremely effective. To demonize him because he was a violent warlord is unfair, because authority is almost always gained and maintained through violence. Genghis was just better at it.