Vicegerent on Earth

I think we cannot go about our willy-nilly matters in life without thinking what is the grand purpose of the oxygens which trees have been so generously give us, the perfect order and proportion of our biological body, and our power to bring change to the earth, unlike any other living beings. So, what is our life for?

QS 2:30 answers the question--to take care of the earth, to be a representative of godly good in the earth.

But I have this unresolved small question: Why the angels compare the previous humans' bad records with their character of always praising and glorifying Allah? Is this comparison strictly about the contrast in behaviour or is there a notion of "we are better for this job" in the question of the angels?