God is love, therefore, it is his nature to heal the sick.  Although it was the Sabbath and Jesus was a law keeper, healing was permitted because it was in keeping with the commandment to love others as you love yourself.  Jesus showed that love is honored everyday of the week.

Also, healing on the Sabbath, Jesus knew this would add to the Pharisees plot to have Jesus arrested. The Pharisees were blinded by their perverted laws, which they referred to as the laws given to Moses. Their objection to Jesus healing on the Sabbath illustrated their blindness.