The old, long-standing maxims, "to the victor belong the spoils" and "history is written by the victors", are not just glib statements.
American expansionism in the 19th century was not generally any more or less altruistic than colonial expansionism (whether British, Spanish, German, or otherwise) elsewhere in the world by other powerful nations. While it wasn't all bad, neither was it all pretty. Too often, significant historical figures are painted in relatively simplistic roles, either good or evil, depending upon who is telling the story.
Looking at and studying history from a more balanced and nuanced position does not have to mean tearing down the heroes of the past. It just means looking at them and understanding them better, warts and all. Putting them in proper context includes understanding societal norms at the time, even while comparing them to modern standards, so that we can see and comprehend the differences. That's a learning experience.
Ultimately, it can hopefully help to avoid repeating past mistakes. That, of course, leads to another old maxim, by Spanish-American philosopher, George Santayana. "Those who cannot remember the past are condemned to repeat it."