I'm not sure I understand... is God defective because He is merciful?
The absurdity of saying that God has a defect is the first objection St. Thomas lists to his teaching that the reason for taking pity is a defect in the person who takes pity:
For it is proper to God to be merciful, wherefore it is written (Psalm 144:9): "His tender mercies are over all His works." But there is no defect in God. Therefore a defect cannot be the reason for taking pity.
He answers this objection briefly:
God takes pity on us through love alone, in as much as He loves us as belonging to Him.
This Divine way of taking pity "through love alone" contrasts with the human way of taking pity "in so far as one looks upon another's distress as one's own."
St. Augustine makes a similar distinction after he proposes (contrary to the Stoics) that our passions, when guided by reason, can move us to act virtuously:
However, it may justly be asked, whether our subjection to these affections, even while we follow virtue, is a part of the infirmity of this life? For the holy angels feel no anger while they punish those whom the eternal law of God consigns to punishment, no fellow-feeling with misery while they relieve the miserable, no fear while they aid those who are in danger; and yet ordinary language ascribes to them also these mental emotions, because, though they have none of our weakness, their acts resemble the actions to which these emotions move us; and thus even God Himself is said in Scripture to be angry, and yet without any perturbation. For this word is used of the effect of His vengeance, not of the disturbing mental affection. [emphasis added]
You won't become popular telling people God has no fellow-feeling with the miserable, because on first hearing that sounds like a defect in God, and few people are willing to give it a second hearing.
Those who think it through, however, will find, not only that is it true that God doesn't have fellow-feeling with the miserable -- and, consequently, that God's way of showing mercy is not man's way -- but that we wouldn't want Him to. (Demonstrating the truth of this statement is, for now, left as an exercise to the reader.)
And while I don't suppose there's any comfort to be drawn from this conclusion, it's still true that it largely settles the amateur atheist's argument from human suffering against God's existence, the one that assumes God ought to be moved to help those who suffer in the same way human beings ought to be moved to help them.