The motive of the war isn't the point, is it? It's whether or not you have statues in honor of those who fought on the losing side. From the standpoint of the royal family, Cromwell is a murderous regicide, beyond redemption. As for the issue of slavery -- Cromwell practiced slavery, enslaving nearly 100,000 Irish and expanding the practicing, sending Irish slaves to the Carribean to the point that Ireland was significantly depopulated.
But again, I didn't think the motive of the various sides in the Civil War was under discussion. The motive of those who erected the statues is under discussion. You think a statue of Robert E. Lee, erected by CSA veterans in 1899 is a deplorable emblem of racism that should be destroyed. I think it's very little different from a statue of Grant erected by Grand Army of the Republic members in the same year -- an object of nostalgia that now, because of its longevity, has a history of its own that shouldn't be destroyed because we've attempted to apply a 21st century sense of morality to 19th century men.