I think you first have to answer, “In what sense is anything objective?”
Certainly, a radical phenomenologist would say that everything is “subjective.” But such a statement does violence to well-established conventions which draw distinctions between subjective and objective, in a way that renders both terms meaningless. This convention holds that “subjectivity” applies to the world as it appears to us, while “objectivity” applies to the world as it is.
To say that we live in a world of subjective appearances does not categorically preclude that there is an independent objective reality that shapes the world as it appears to us. Indeed, the term “objective” refers to this latent structure of the world.
Nobody has ever seen an atom, yet in the totality of our knowledge, we can be reasonably sure that they exist. Why? Because we can specify rigorous experimental proofs. In other words, we can contrive a set of circumstances in which the world of appearances behaves, with convincing regularity, in a manner that can only be reasonably explained by a specific underlying reality. In that sense, convention allows us to speak of atoms as objective facts—as opposed to fantasies, opinions, impressions or suggestions.
In other words, “objectivity” refers to the verifiable latent structure of the manifest world, as revealed not in appearances but in the totality of our knowledge. Just as we can infer the existence of atoms organizing our experience of the material world, so too we can infer the existence of things like “norms” and “mores” as the objective organizing principles of our social experience. In fact, these norms and mores are so real to us that we are willing to go to war to preserve or abolish them.
So, notions of “good” and “evil” to the extent the are embodied in norms and mores, can be said to be “objective” insofar as they organize our perceived social experience.
To say that good and evil are only “conflicting interests, opinions and goals,” is like saying that a mugging is a conflict of interests, opinions and goals between a mugger and his victim. It is true, but by no means the whole truth in the rich totality of our knowledge about the norms which encode our moral notions of good and evil.
When you say that good and evil are merely subjective concepts, you effectively deny that there are any underlying norms and mores that shape our experience of them. Good and evil are grounded in survival issues. For example, we generally describe things that promote our survival as good, and things which threaten our survival as evil. Accordingly, acting in good faith preserves the social fabric and social capital; acting in bad faith destroys trust and social cohesion and threatens the survival of the group and everyone in it.
Slavery and cruelty are not evil simply because they offend our sensibility, they offend our sensibility because they are objectively evil. They destroy the independence of individuals which is a necessary condition of collective intelligence of our mass market-based culture. In other words, acts of bad faith impinge adversely on the “wisdom of crowds” phenomenon in ways that directly threaten the survival of our market-based society and culture. Moreover, this was always the case, even when it was not apparent to slave-holding barbarians in ages past.