Right Intention, Wrong Way

Rabbeinu Bachya also gives a Kabbalistic reason. Nadav and Avihu saw fire descend from Heaven upon the mizbeach, and they understood that it came from the supernal place of Judgement. They feared that this Judgement would pass through the camp with devastating effect, so they ran to offer the incense, which was ordained to soften the forces of Judgement. Their error was in the method. One cannot appease the forces of Judgement directly, for who has the power to appease the wrath of Hashem? One must instead call upon the essential Name Havaya and depend on Divine mercy, rather than approach through other channels. They did the right thing in the wrong way: their intentions were good, but their action was not.