ORIGINAL: Yogi the Great
Ah yes the old two wrongs make a right argument.
But it does illustrate the pitfalls of applying 21st Century morals to 19th Century situations.
The Union was carrying out genocide against the Sioux in the Dakotas even as they were prosecuting their sanctimonious crusade against slavery. And after the war they cranked it up a bunch, with one Union general (Sheridan) famously stating "The only good indian is a dead indian". And it wasn't just targeting warriors. It was true genocide that included women and children. As Colonel Chivington famously stated after the Sand Creek massacre, "Nits make lice". And don't forget handing out smallpox infected blankets and starvation via extermination of their buffalo food source.
And those two wrongs were not equal. Genocide is rightly considered far worse. Note that the Nazis had slave labor camps in WWII, but they barely warrant a notice since their genocidal crimes trumped them.
Now, don't misunderstand me here. I'm not actually castigating the Union for their actions against the indians. The reality was that the indians were the instigators of genocidal warfare - that was how they rolled. What I'm saying is that the morals of the 21st Century can't be applied to the 19th Century.
We live in extremely lush times of plenty. We can afford the luxury of considering genocide and slavery to be evils. But times can get very hard. If the margin between life and death ever became very thin again, our morals would be revised accordingly. I like to use the example of the Donner Party to illustrate that times can get so hard that you'll not only exterminate your neighbors, you'll EAT them, too!
Mid-19th Century Americans lived in a land that was only recently converted wilderness (if even that). They faced threats to life we can't even fathom. Who are we to question their strategies for survival? That's especially true if you're a descendant - if they hadn't survived you wouldn't exist.