The impact Columbus had on the "New World" depends on the perspective you are using. From the perspective of later European migrants who would settle there, it is arguable that Columbus's impact was positive because he provided confirmation about the location and wealth of the Americas.
From the perspective of Native peoples (and specifically the Arawaks), his impact was undoubtedly negative. Columbus arrived in the New World looking for wealth to bring back to Spain in order to justify his voyage and, as he hoped, a larger second voyage. In order to obtain this wealth, Columbus demanded that the Arawaks deliver large amounts of resources. This forced the Arawaks to labor for long hours and for days on end and prevented them from sustaining the agricultural and hunting lifestyle they had led before. Columbus and his men used the Arawaks as slaves, working many to death, beating some to death, and killing many more through the introduction of new diseases for which the Arawaks had not developed resistances. Some of the tortures that Columbus used when he was not provided his demanded quantity of resources foreshadowed the techniques used by Leopold's men in the Congo several centuries later.
For the people that resided in the Americas prior to Columbus's landing, his impact was devastating. For those that would come after, Columbus's impact was arguably positive because it eased later conquest.