So...you have the power to give justice. The justice you deliver should be the result of an unemotional decision mixed with common sense. If a judge forgets about mercy there is a chance that his/her decisions will develop into a form of tyranny.
This seems to be a warning given to new judges to not only follow the letter (as written) of the law, but also, the spirit (the intent) of the law. If a person steals a loaf of bread because he is hungry, should a judge decide that this person should go to jail for six months (the term imposed by the current law - the letter of the law), or should the judge allow the person to go free and only force him to pay for the bread (a show of mercy)? If a judge merely applies the law and puts people into jail for minor offenses, the jail terms given (the justice) will become, over time, a sort of tyranny.