Generals of any war are a part of history so they deserve to be learned about and remembered. Humans tend to celebrate anniversaries and make statues in order to remember history.
In your example the civil war in America was historically more nuanced than just the controversy of slavery therefore some generals might have been fighting the North with slavery being an ancillary reason.
Modern people shouldn't judge a person or persons living in the past based on contemporary standards. If we do that than we should equally not honor the generals fighting for the northern side of America's civil war since they were defending states that later would tend to adopt educational segregation and miscegenation laws after the civil war such as Ohio, Indiana and Rhode Island.