If you don't think slavery is morally wrong, you've screwed up. You don't need to interrupt everything you do every five seconds to remind people that you know which things are good and which things are bad. It's actually pretty much okay to depict a thing without specifically going out of your way to identify whether you think it is right or wrong, and let the reader think about it.
I'm not proposing "instead, depict it as neutral" or "instead, depict it as positive". You can simply not address the question of whether a thing is morally good or bad because not everyone wants/needs to be handheld through the moral judgment process every minute of every day. You can just have a story in which some people are murderous and others aren't, and not specifically stop to say "by the way the murderous ones are bad".