The sins that are likened to a menstruous woman are not so severe, for they are sins that are implanted in man's soul and are part of human nature. Such transgressions are not severe enough to be regarded as a corruption of the soul that totally veers from human nature. Sin that is likened to a menstruous woman testifies to human weakness rather than moral corruption.
The cyclical nature of the uncleanness of the menstruous woman which diminishes its gravity also aggravates it. It is not a one-time uncleanness that disappears over the horizon after the woman purifies herself, but rather uncleanness that is built in to the female body, and reappears periodically. Just as it is clear to us that the woman will return to her clean state, so too is it clear to us that she will once again become unclean. The fact that these sins are implanted in man's soul indicates not only that they are less severe, but also that they are likely to recur. A person cannot overcome them and make them disappear, but rather they remain as part of his fixed conduct. The hope that the people of Israel will internalize God's will and change their nature by making their own will correspond to His will fails to be realized.
However, there are sins that are likened to the impurity of a dead corpse. While it is true that the uncleanness of a corpse is accidental, rather than expected or built in to nature, it is, however, more severe than other types of uncleanness, for it testifies to the total failure of nature and matter to maintain themselves, and to the cessation of man's capability of breaking out of his world. It therefore requires stronger atonement, namely, the atonement of the red heifer.
It differs in its very essence from the purification process of a menstruous woman. A menstruous woman immerses in a mikve filled with water in its natural state and in abundance. A reservoir of rain water or a natural spring can be arranged in most places. In contrast the "waters of sprinkling" (mei chatat) that purify a person from the uncleanness of a corpse are extremely rare and precious, and not available in all places and at all times. More importantly, they do not achieve cleanness through a restoration of the natural state, for the uncleanness that they wish to remove does not reflect a deficiency within the framework of nature, but rather a failure of the natural world in its entirety. This requires a purification process that breaches the boundaries of nature and repairs it by connecting to elements found beyond it. Since we are dealing with a corruption of the entire natural system, it is necessary to destroy a natural object and rebuild it.
The same applies to sins that are likened to the uncleanness of a corpse. The prophet relates to such sins with extreme harshness. We are not dealing here with an ordinary sin, but with a sin that testifies to moral or religious corruption, and with a person who veers from ordinary disobedience. While it seems that they are not subject to repair, and that repentance on the natural plain cannot repair and achieve atonement for them, the mystery of repentance that involves God's purifying man is capable of removing the sin.