See, you are so steeped in your position that you can’t possibly understand why calling either side of the participants of the Civil War “immoral” might offend people, especially people who, for the most part, agree with you.
Both sides of the American civil war were "immoral".
But I think the one that invaded and killed people were the more immoral. I also don't believe they did so for moral reasons. I think they did so for reasons of power and money.
And in that regard, I think it greatly resembles what Russia is doing to Ukraine.