Didn't they also shave off the ambassador's beard or something?
I think I read someplace that such an act was a sign of terrible disrespect.
I don't recall; I think you're right, though. This was the second ambassador that the Persians killed.
from http://library.thinkquest.org/11847/gather/9b.html, a pretty good summary of what happened:
"In 1209, Ghengis Khan's invasions began. He began by taking Xi Xia, a kingdom situated along the Silk Road that exacted heavy taxes from Mongol caravans. His next target was Jin. However, he was unable to attack over the 40-foot walls which confronted him. So he instead ravaged the countryside for several years. When he returned in 1214 with 70,000 troops and the Chinese technology of mangonels, an advanced catapult that could fling 100-pound weights, the Jin quickly fell, offering gold, silver, and a princess in tribute. A short time later, the Jin began to regroup further south. Ghengis Khan quickly attacked again, slaughtering a great number of the Jin people. In 1218, 20,000 horsemen easily took Kara-Khitan, where the mostly Muslim population had been religiously oppressed. The word of the Mongol army began to spread. "In the countries that have not yet been overrun by them, everyone spends the night afraid that they may appear there too" (Ibn Al-Athir, NG 96, p 9). "
"Ghengis now set to work on building his capitol city, Karakorum, with help from the thousands of artisans he had collected from conquered realms. He also built a government with the help of Chinese scholars. He attempted to open up trade with neighboring Samarkand, sending 450 merchants to the capitol. A border governor arrested them and had them all executed as spies. When Ghengis demanded [of the Shah -Spk] that the governor be handed over for punishment, the ambassador's head was sent back in reply. This was a supreme offense to the Mongols. "In retribution for every hair on their heads it seemed that a hundred thousand heads rolled in the dust" (Juvaini, NG 96, p 28). They asked for more troops from Xi Xia, but were turned down, an affront that would be revenged. With 110,000 men Ghengis surrounded both major cities in the kingdom, bombarding them with the mangonels for a month before they fell."
Ghengis had gone out of his way to make the first embassy palatable to the Persians, to the point of ordering that the ambassador and staff sent should be Muslims. By doubly insulting Ghengis, the "civilized world' announced that they thought they could slaughter him and his at will. He probably figured "it's them or us, and I'd rather it be us" - so he took on the world.
One of the "unexpected consequences" of this was the siege and sack of proud Baghdad in 1258, well after the death of Ghengis. An interesting aside - Ghengis introduced many modern concepts to warfare, among which was this rule: 'It is forbidden to abandon any living comrade in the field of battle; at pain of death.'
Ghengis was absolutely not evil. Ruthless in war, yes. Deliberately cruel? No. The general picture of him as a demon is colored by the accounts of the people who insulted him and defied him.