The principle of the rule of law transcends mere laws.  For example, a law that specifically stated that disobeying court orders was a crime except if done by police officers would violate the principle of the rule of law.

There's nothing wrong with pardons for compassionate reasons. The problem comes from pardoning the powerful - such as an unrepentant police chief.

Trump may not lose any votes over this, certainly not from his base.  I understand President Duterte is quite popular among his base in the Philippines, too.