The question of an absolute moral standard came up, and I know C. S. Lewis among others uses the existence of this absolute standard as evidence for the existence of God. This argument assumes that when we say "That's wrong" or "I'm not perfect" we must be referring to some absolute standard of "right and wrong" or "perfect" that actually exists and that we fall short of.
I don't think so, actually.
I'm a singer. Quite often after a performance, even a performance that went well and that I'm happy with, I'll have the thought, It wasn't perfect. Of course it wasn't. There is no such thing as a perfect performance. Even professionals at the Met do not give perfect performances. So when I say it wasn't perfect, what am I measuring myself against? My own self-determined notion of what I wanted my performance to be. Maybe I ran out of air and ended up having to breathe in the middle of a word. Maybe I was flat on a couple of notes. Maybe I wasn't as expressive as I could have been. Maybe my vibrato wasn't consistent.
These are all things I want to work to improve. Perhaps my unfortunate audience even suffered a little because of my shortcomings, but this doesn't mean that I'm an inherently "bad" singer, or that I need the "grace" of some music god. It means I need to practice. And with practice and guidance, I will get better (I can almost hear the objection, "How can there be better and worse without an absolute?!"), but even after a lifetime of practice, I will not be a perfect singer, because there is no such thing. And no amount of praying to Maria Callas or "faith" in my voice teacher or pedagogy books or anything else is going to impart the "grace" to make up for my vocal imperfections. There is no absolute vocal perfect, just as there is no absolute moral perfect.
You say grace is undeserved. I think a better question than whether or not grace is deserved is whether or not it's needed. The whole idea that we are inherently defective and sinful is one of religion's great harms. No, we are not perfect, but there is no such thing, not even in the order of the cosmos (the earth is not perfectly spherical, as an example I can think of off the top of my head).
Right and wrong is something we determine ourselves. Murder is wrong because we say it is, because we care about life, not because any god out there cares. We are not perfect, but we are continually striving to be better. You do not need any absolutes for there to be better and worse. And the fact that we sometimes fall short even of our own expectations of ourselves does not mean that these expectations are not valid and not worth striving for.