Although it could be fun for, say, a Diablo style game, it's completely unnecessary. Just say, "In this setting and game, goblins are inherently evil chaos spawn with no redeeming qualities unless you hate babies and dogs." Nobody can make killing goblins by the dozen ethically problematic except you.
Morality is a strange thing. I played a the kingmaker computer game recently... I had no problem playing the mite against the kobold and having both groups slaughter themselves (with my enthusiast contribution)... and I found a location where a few mammoth were grazing. 180 xp apiece. I attacked them... paused. Reflected. Reloaded.