Why is the indictment of a former President such a divisive topic in the United States?

You need to look at both the micro and macro implications to fully understand this topic. The micro implications are very specific to the case(s) being brought and therefore won't be covered here. The key macro implication of indicting a former President is the risk that the criminal justice system becomes a tool to persecute political opponents (former, current OR potential) by whichever person or party is in power. There are several examples of this in other countries throughout history. That being said, there are just as many examples of countries indicting former presidents without any sign of abuse. In fact, it can be an important part of the "checks and balances" of a democracy (see France, South Korea, etc.) One of the factors you can look at when determining if the justice system is being abused or not is the number of cases/indictments being brought against the former head of government. When the number of cases is low, there is a higher likelihood of abuse. But when the number of cases is high and for a diverse number of reasons, the likelihood of abuse is lower.