I didn't read this whole thread, so if someone already said this, forgive me. I don't think killing defenseless creatures is evil if those creatures are evil. You see in D&D, good and evil are ontological realities, existent entities, and not just viewpoints. This is attested to by all the detect spells, and by the MM entry for alingment that says always evil.
Creatures in D&D either ally themselves with one of these alignment forces or are intrinsically connected to them by their very being. Demons for example, are intrinisc allies of evil. Creatures with good alignments are allied with good. For a good creature to kill a baby demon is not an evil act; in fact, it is a required act because of that creatures alliance with goodness.
Now Kobolds are only usually evil so it isn't quite as black and white, but the same logic still applies, so a good creature is perfectly within his rights to kill baby kobolds.