I think what makes life precious is the sole fact of it existing. By the sole fact of being, it enriches the universe in a way inanimated matter does not. It's not like if a life form is not intelligent or doesn't moan it's ok (or pretty or progressive) to destroy it. I don't really know why Micheal went onto murdering to decide whether or not something would be intelligent. I'd say anything that would be capable of staying alive and capable of respecting other life forms deserves to be left alone, regardless of anything you may think of it.