I agree that the ending was brilliant, in spite of the spoilers ... but my reason for thinking it was effective seems to be unique to me.
Shane isn't the Darwinian one, at least not in a conscious way. He's all for the hard tough-guy decision if it means killing anonymous walkers; he's as paralyzed as everyone else when it's Sophia, and it means re-killing her in front of her mother. In that sense, Herschel has the moral victory: his theory that there's a cure seems unlikely, but his point that walkers are our mothers, spouses, children, etc.? He's spot-on about that, and the hesitation of every one with a gun when Sophia lurches out the door testifies that Herschel is right, that there is a moral quandary here.
And then there's Rick, who answers Lori's faith that he is the kind of man to lead them, that Dale is wrong that this world is meant for someone like Shane. This world needs people like Rick, who can reinterpret the firing of a bullet as an act of compassion and not just fear or machismo. It also, paradoxically, points to the major theme of the last few volumes of the graphic series: can Rick make these decisions and still keep his soul?