Are justice and mercy incompatible?

Justice and mercy are not incompatible. 

Human justice and Divine justice have little in common besides the principle.

Divine justice is based on absolute knowledge and takes into consideration all the circumstances.

For instance, when you leave earth, no court sits in judgment—no sentence is passed. The only judgment you must submit to is the inevitable result of the law, which brought you into being, and which declares that every act and every thought of your earth life sets in motion waves or impulses either good and harmonious with their divine origin—or bad and inharmonious. 

Since you have a soul, which is a part of that great Creative Spirit, and which must ultimately come into perfect accord with that Creative Force or Spirit, it follows that the waves of dissonance must be accorded.