Why did the mongols see the need to kill almost all of the inhabitants of nishapur?

Mongols kill more than 1.5 millions of people of Nishapur due to Genghis Khan's conquest tactics during that time.
Before attacking a city, he always give the time for that city to surrender and serve him (and required to pay taxes) or he and his army will massacre all people in that city and loot them, and the people of Nishapur chose to stand up and put up a fight.

Gengis Khan needed this to conveys some points to other cities:
- His army has the destructive power that could cause serious damage to them.
- He is not playing around with his threat and set the nishapur as an example
- It also serves as psychological warfare for the other cities, because they will lose their willingness to fight Genghis Khan before it even started