First thing is, there is no such thing as race, we are human beings, Race is a social construct that divides people for no real reason. So then arguing that racism is bad only makes sense if you accept the fiction. What race is a person who is the child of the many fictions that are race. So the simple answer is racism categorizes people and defines humans due to meaningless categories. It divides and in the case of hate makes discrimination and hate possible.
"The main hypothesis concerning group-think is this: the more amiability and espirt de corps among the members of an in-group of policymakers the greater the danger that independent critical thinking will be replaced by groupthink, which is likely to result in irrational and the dehumanizing actions directed at out-groups." Irving L. Janis in 'Sanctions for Evil'
'In the biological and social sciences, the consensus is clear: race is a social construct, not a biological attribute. Today, scientists prefer to use the term “ancestry” to describe human diversity (Figure 3). “Ancestry” reflects the fact that human variations do have a connection to the geographical origins of our ancestors—with enough information about a person’s DNA, scientists can make a reasonable guess about their ancestry. However, unlike the term “race,” it focuses on understanding how a person’s history unfolded, not how they fit into one category and not another. In a clinical setting, for instance, scientists would say that diseases such as sickle-cell anemia and cystic fibrosis are common in those of “sub-Saharan African” or “Northern European” descent, respectively, rather than in those who are “black” or “white”.'
https://sitn.hms.harvard.edu/flash/2017/science-genetics-reshaping-race-debate-21st-century/