Justice is not necessarily the same as Law. True justice seeks out the spirit of the law, not just its letter. If a law is bad then true Justice will set that law aside. This is the sacred responsibility of those given the power to judge.

If well aspected in a Tarot reading, this card can indicate settlement of disputes, the achievement of a just outcome.

If badly aspected this card can indicate corruption and failure of justice.

Judge each case on its merits.

While this might not be the truest meaning of the Justice card, it certainly is the best for Supernatural. After finally catching up to the thing that killed their mother and Sam's girlfriend, Justice is finally served. Dean uses the gun made by Samuel Colt, one specially crafted and blessed so it can kill demons, to put an end to the plans that the Yellow Eyed Demon has plotted.

The Justice here isn't imparital but it is just. The hunt that has taken a lifetime and stripped the boys of everything that is meaningful to them is finally over.