Tikkun 21 – 58
The Red Heifer purifies the impure but brings impurity to the one that makes the purification process.
The same situation happened with the water process for the woman who was accused by her husband of infidelity.
The priest became impure when he worked with the water.
It looks like an illogical situation and the Zohar explains that the impure was pure before and was from the right side of Chessed. To purify the right, the priest brings the light of Chokmah, which is the left column. When the left gets mixed with the right, it creates balance and the impure becomes pure. Since the priest had to use direct light of Chokmah to remove the impurity, it brought judgment on him. The priest had to purify himself through a different process.