The positive and negative impacts of Hernan Cortes depend on the point of view from which you are looking. From the point of view of a Native American from what is now Mexico, it would surely be very hard to find any positives about Cortes. From the point of view of a Spaniard, or perhaps a Mexican of certain political views, it is more possible to see Cortes in a good light.
It is easiest to look at Cortes and see the negative effects of his actions. By his actions, he conquered the native peoples of Mexico and subjected them to Spanish rule. This meant that the natives lost much of their native culture. It meant that they came to be a subordinate people in what used to be their home. It meant that they were exploited for their labor and generally treated quite badly. In short, we can see Cortes as the symbol of imperialism and colonialism that destroyed the native way of life in Mexico.
If you take a more Eurocentric view, then you can argue that Cortes had positive impacts. You can say that the actions of Cortes and his men brought civilization to Mexico. You can argue that, by overthrowing the Aztec, Cortes put an end to a terrible regime that practiced such things as human sacrifice. You can say that Cortes and the other Spanish conquistadors allowed Mexico to become a modern nation.
Thus, one’s view of Cortes’s impact is very much conditioned by the point of view from which one examines his actions.