Mercy and the Law are not mutually exclusive.  True mercy is doing whatever we can to help people rise above their sins and get to heaven, and the law provides us with a road map of how to get there.  We should do whatever we can to help the less fortunate in this lifetime.  We should also encourage them to act in such a way that they will achieve eternal salvation.  It is not love to tell an alcoholic that he should drink beer whenever he feels like it in order to be "true to himself".  It is not love to "accompany" a kleptomaniac  by telling them that stealing stuff is not such a big deal.