If something causes more harm than good, then it's unethical. Otherwise, it is not.
Demonstrate that, by reference to material physical reality. That this doesn't work isn't new, Hume thoroughly demonstrated this a long time ago.
... the gene for compassion.
What? Where is that in our DNA, specifically?
And the origin for this rule is three billion years of evolution: In the long run, we will survive as a species if we can build cooperative and well-functioning societies.
The origin of a drive vaguely correlating, in your most hopeful case, to something touching on something related to feelings about something in ethics, maybe. That isn't ethics. That's instincts. As well as, anyone simply stating "Well, your feelings may lead you to feel that way, but my feelings lead me to feel the opposite, so evolution must be validating me rather than you" completely negates your argument. Not to mention the actual history of evolution, in which intertribal -competition-, not cooperation, primarily formed our human attributes. That's why it is completely dysfunctional as a basis for ethics. Evolution only gives you broad instincts, that is, feelings, which may or may not happen to correlation to the correct course of action in a particular case, and they are nowhere nearly fine-grained enough to deal with complex issues on an a priori, axiomatic basis.
It's an enticing thought, and given that it's difficult to even tolerate acknowledging the fact you have no basis for you ethics you can name, and are therefore self-declaredly entirely amoral, and -any- ethical happenstance you have to have, is -totally- owed to cultural assimilation from -religion-, the worldview you deny, and as you do so invalidate yourself... it's understandable on your part.
It just doesn't actually work in any way or have any validity, is all.