What would be the harm?

[Angrboda]

Plenty of things that don't adversely affect some persons interests are harmful. 

Since you have yet to provide a valid example of such, this is simply an unproven claim. You even acknowledge the possibility that having the shitty morals of valuing personal interest against that of a generalizable interest might lead to the extinction of such persons. There can be no greater selective pressure than that of extinction. If you aren't alive, then you most certainly can't pass on your genes. (You need to show that people who don't forbid such things at least break even in the genetic race with those that do. And we have plenty of reason to believe that they don't.)

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This raises a side of the issue that I tangentially explored with my questions about empathy.  If our morals are a product of evolutionary processes yielding determinative conclusions about the right or wrong of a thing based upon our contingent history as biological beings, what do we do once we realize this and can hypothesize answers that lie beyond the dictates of our biology, and on what basis do we make conclusions then, released from their mooring in our evolutionary history?  Can we out think our biology, and if so, how?

Again, it depends which ones of those archetypal demons are most at work, either in people's natures or by training to match perceived societal norms.

I'd say in general we seem to have some kind of homeostatic mechanism-- one generation tilts toward anger, outrage and war, the next tilts toward peace and restraint.  Surely one generation cannot suddenly have a superior moral mechanism than its immediate predecessor.

That's nice. Not relevant to the question, but okay.

I think the freedom you are talking about leads to another set of philosophical problems: if self-realization of evolved moral instincts frees us from them, then what next?  Do we go full-on Machiavellian / Ayn Randian / LaVeyan?  Or do we develop a much greater capacity to understand, forgive, and cooperate with others?

Both of those options are rooted in biology. Without biology, we have neither impulse. As Dostoevsky said, “If everything on earth were rational, nothing would happen.” Our interests, whether altruistic or selfish, are rooted in biology. The question is how to transcend them. If we abandon our biologically rooted motivations and methods, the only thing left to appeal to is reason, but reason is neutral in the matter, and does not prefer one way t' the other. If we depend upon reason alone to move us to action, we simply will cease to act.