Why did the Mongols launch the deadly attack on Nishapur

In the beginning, the Mongols were a peace-loving, pastoral people who only wanted to be happy in their own lands. However, Muslims started invading their lands and waging wars which obviously they were not okay with. It culminated in the city of Nishapur where Muslims revolted against the Mongol rule. This is why Genghis Khan's daughter sentenced everyone to death, which is how thousands of people were murdered.