Here's a bit that explains the necessity of using judgment in the sense of discretion and wisdom...along with compassion.
The union, often called mercy, is also described as focused compassion.
I. #4 Love (lovingkindness, compassion)
Chesed is associated in the soul with the desire to embrace all of Creation and bestow upon it goodness. As the expansive force which impels the soul to connect with outer reality, chesed inspires, and thus implicitly accompanies, all the other expressions of emotive force which succeed it in the soul.
II. #5 Might (judgment, restraint)
Gevurah is associated in the soul with the power to restrain one's innate urge to bestow goodness upon others, when the recipient of that good is judged to be unworthy and liable to misuse it. As the force which measures and assesses the worthiness of Creation, gevurah is also referred to in Kabbalah as midat hadin ("the attribute of judgment"). It is the restraining might of gevurah which allows one to overcome his enemies, be they from without or from within (his evil inclination).
Chesed and gevurah act together to create an inner balance in the soul's approach to the outside world. While the "right arm" of chesed operates to draw others near, the "left arm" of gevurah reserves the option of repelling those deemed undeserving. (Even towards those to whom one's initial relation is that of "the left arm repels," one must subsequently apply the complementary principle of "the right arm draws near").
Ultimately, the might of gevurah becomes the power and forcefulness to implement one's innate desire of chesed. Only by the power of gevurah is chesed able to penetrate the coarse, opposing surface of reality.
III. #6 Mercy (beauty, truth, balance)
Tiferet is associated in the soul with the power to reconcile the conflicting inclinations of chesed and gevurah so as to allow for focused compassion, thus accounting for its designation in Kabbalah as midat harachamim ("the attribute of mercy"). The beauty of tiferet manifests itself through the elegant blend of emotive gesture implicit within its expression.
Peace & blessings,
7L