Jesus shows the inconsistency of being willing to devote more care to an animal than to a human being, who is far greater than an animal. Having shown that you may heal the sick on the Sabbath, Jesus rounds off the discussion with the general principle (Matthew 12:8) that it is lawful to do good on the Sabbath. To neglect doing good when one has the chance is evil (see Mark 3:4; Luke 6:9), and to do evil would be the greatest desecration of this holy day.
Jesus highlights their inconsistency: that they were willing to devote more care to an animal than to a human being, even though man far surpasses an animal. Having shown that it is permitted on the Sabbath to heal the sick, Jesus now decides to round off the discussion once again by means of a general principle (see Matthew 12:8): that it is lawful to do good on the Sabbath. For if this were to be forbidden, the day would be an evil day. To neglect doing good when one has the opportunity is itself an evil deed (see Matthew 12:8; Mark 3:4), and to do evil would be the greatest desecration of this holy day.1
12 Combien un homme ne vaut-il pas plus qu'une brebis! Il est donc permis de faire du bien les jours de sabbat.