Morality can theoretically be objective without coming from an objective source. Under Kantianism, the principle of universality is used. If an action cannot be universalized, then it is bad. Stealing is bad because if everyone stole there would be no property. Lying is bad because if everyone lied there would be no trust. You could theoretically take issue with things like "trust" being objectively good I guess, these are the problems with Kantianism, but most don't. It's objective in the sense that anyone can come up with the same principles by universalizing, essentially making sure they don't make exceptions for themselves.
This is an outlook I've always liked. It doesn't work in every circumstance but it is a good basis to work off.
Aren't those things just despicable in your opinion though, right? I mean myself and almost everyone else would agree with you, but that doesn't mean the statement "hurting people is bad" is objectively true. The most "we" as the majority in society can say is "we don't like people getting hurt" and then we can go about acting on that.
Well, that's kind of what I'm saying, that it's not possible to find actual objectively true morals, but we can work off broad principles. We can say that things like that are bad because of the psychological and similar suffering it causes in their victims, and, regarding the principle of universiality specifically, if society was such that everyone was simply allowed to molest children, and rape people as they pleased, that would cause an immense amount of suffering. And we can get to the idea of "hurting people is bad" because it can almost be universally agreed that being hurt (through undesired suffering) is undesirable, what it means to be hurt depends on the situation and the individual.
Autonomy is also something to take into consideration.
If everyone just suddenly lost all empathy and started killing each other there wouldn't be anything "wrong" with it because there wouldn't be anyone around that felt it was wrong.
Well, I think you can look at how the one being killed felt, or would've felt about that. If everyone started killing each other, but everyone was also okay with being suddenly murdered then hypothetically, I suppose there wouldn't be anything wrong with it. But if people are being killed without their consent, and still maintained the feeling toward being killed that exist in the humans in this reality (fear toward death, avoidance of physical pain, etc) even if they wouldn't have empathy for it being done to someone else, then surely they wouldn't want to experience whatever gruesome death awaits them for themselves. In that way we could say that something is wrong as the suffering is still increasing.
In the BDSM example, I don't see any contradiction as the fact that physical pain is being had is what directly leads to the increase in happiness. They don't want suffering, they want happiness, and it just so happens to route to that is something that most people would not find enjoyable, ie physical pain. For example, tattoos tend to lead to physical pain, yet it causes an increase in happiness for the person getting the tattoo, so it wouldn't make sense to deem the tattooer as commiting an immoral act.
I think suffering being defined as undesired pain is what makes most sense, but of course there are always things that muddy the water.