First, Christobal Colon (Columbus) was an explorer and adventurer hoping to find a westward route to India, to make trade simpler and more profitable. He wasn't a slave trader, but did bring captive natives back to Spain, possibly introducing a potential source of slaves. All expedition was motivated by profit, not just some desire to advance knowledge.
Second, there is some speculation that Colon was of Italian Jewish descent and that his voyages were in part a search for safe haven for the Jews of the diaspora. He sailed on his first voyage in 1492, the same year as the Spanish "expulsion ", when the 800 year "reconquista" was declared won and non Catholics were ordered out of the country. Many Jews became catholic in name only to avoid persecution by the religious authority under the Spanish Inquisition (the conversos), and some of these took the first opportunity to distance themselves from the religious authority.
During the Spanish civil war, many churches were burnt to the ground, and with them birth and historical records, so a large portion of Spanish history was simply destroyed and beyond the reach of researchers and historians. Those who are deeply offended by the history of slavery tend to vilify everyone associated with its history except the natives involved in selling captured neighboring tribesman into slavery as profit from their warfare. You don't hear much about the Moroccan slave traders, the role of the Ottoman empire in the slave trade, or that of the Moors.
The current social justice movement is primarily concerned with the western colonial powers and their role in slavery and indigenous exploitation, not the history of slavery as a whole (which spans all of history as part of the spoils of war,)
Hernan Cortez is more well known as a conquistador and adventurer that did wage war against the indigenous peoples of central America for profit. The poorly educated might simply lump Columbus together with Cortez, Pizarro, and the other government sponsored conquistadors, because he made their conquests a possibility through his "discoveries." That's a bit like blaming Albert Einstein for the destruction of Hiroshima and Nagasaki.
Historically, the conquistadors were responsible for more death through disease than through warfare, by introducing viruses that were new to the "New world" like syphilis and acne vulgaris (common acne was originally a plague that took many lives, but left the surviving population with natural immunities.) Angry liberals are less concerned with facts than feelings and so we have new and distorted perceptions of history.