They had many parts in hurting the world one of those is the silk road. This was not good because it ended up transmitting a disease into Europe the bubonic plague. This plague ended up killing one third of the entire European population. This would not have happened if the mongols did not use the silk road.
Another reason was their conquest. If a village or area did not surrender they killed everyone and burnt down the village. They also plundered from homes they were burning. Because of this more than a million people were killed.
The final reason that the mongols had a negative impact on the world