Even if a communist can colloquially describe themselves as being on the left, there’s a distinction between communism and “the left.” This is implied right in the title of Lenin’s Left-Wing Communism: an Infantile Disorder. Whereas the left, a big tent term for a myriad of incompatible ideologies, aims merely to act as an opposition towards the present order for the sake of it, communists have a coherent vision for how to defeat the system: by advancing history’s development to the next stage. The left, because of its lack of commitment to that central Marxist goal, naturally takes on an opportunistic role. Because when you want only to build a movement as an end in itself, rather than use this movement as a means for defeating the system, you become nothing more than an actor who benefits from discontent without helping solve the problems behind that discontent.
For describing what was happening at the time, it was the progress of capitalism maturing, based on the material conditions and over time we increased what humanity was capable of. Was the slavery necessary? No, and the civil war allowed the industrialists to move south and helped capital expand more. Was the genocide necessary? Probably not, but that alternate timeline would have had white and indigenous people working the goldmines and striking together against indigenous capitalists. We are not living at that time or timeline and can recognize that we need to make amends to our black and Indian brothers and sisters.