Our Gemara on Amud Aleph brings the opinion of Rabbi Yose who holds that in many situations we say, one prohibition does not fall upon another.  For example, if one were to engage in sexual relations with a woman who is forbidden on two counts, such as his mother in law, who also happens to be married, he incurs the penalty of the first prohibition that was activated even if the second prohibition has a more severe punishment.  The penalty for relations with a mother in law is Sereifah (burning), while the penalty for relations to a married woman is Chenek (choking), which is less painful and less severe.  Thus, if at the time he married, his mother in law was already married, the first activated prohibition was that of a married woman, since she was forbidden before he became related to her as his mother in law.

Simply understood, the idea that one prohibition does not fall upon another is a legal mechanism.  However, we must understand the Torah value and lesson in this legal technicality.  After all, from a point of justice and fairness, he did actually have relations with his mother in law, so why should he not incur the greater punishment? My impression is that there is some overriding idea about compassion that the Torah is trying to teach.  Maybe it is to help a person who commits multiple sins bear up better under the guilt.  It reminds me about my parenting errors in my youth.  If my son and I were having a bad day and getting into power struggles, he might get docked or punished for so many infractions that the entire process was undermined. “You have to go to bed 10 minutes early, no 15! Wait, you are smirking, I’ll add another 10 minutes.  Forget it, just go to bed now!”  There is something compassionate and merciful about the idea that the person can only be held liable for one prohibition, even when at times, he violates many.

The Arvei Nachal (Noach 3:43) also discusses this and I will offer my interpretation of his cryptic words. Let me first quote him:

ארז"ל (יבמות ל"ב.) מי שנתחייב ב' מיתות נידון בזיקה הראשונה הבאה עליו, וזה כי מעשה האחרון שעשה נמשך מהראשון כי עבירה הראשונה מחייבי מיתות בית דין דנעשה גברא קטילא ולא היה לו שוב שום שמירה לכן אין להענישו כלל על מעשה האחרונה, וכן הוא בתחלואי הגוף כי אם נתחדשו באדם חלאים רבים לא ישתדל הרופא לרפאות רק החולי אשר סיבב כל החלאים האלה כי בהתבטל הסיבה יתבטל המסובב ויתרפאו כל יתר החלאים מעצמם.

Our sages stated that one who incurs two death penalties, is only judged by the first prohibition that was activated. This is because the last sinful act he did was really a result of his first. Once he incurs the death penalty, he is essentially a dead man, and “no longer has protection”, so he is not liable. This is comparable to a man who is severely ill and then contracts an opportunistic infection. The doctor treats the original illness in order that he will cure it by getting to the source.

The difficult to comprehend part of the Arvei Nachal is, what does he exactly mean by “no longer has protection”. It seems as if he is referring to losing some kind of spiritual protection as a result of his sin. This is reminiscent of the Gemara (Kiddushin 30b):

And Rabbi Shimon ben Levi says: A person’s inclination overpowers him every day, and seeks to kill him, as it is stated: “The wicked watches the righteous and seeks to slay him” (Psalms 37:32). And if not for the fact that the Holy One, Blessed be He, assists each person in battling his evil inclination, he could not overcome it, as it is stated: “The Lord will not leave him in his hand” (Psalms 37:33).

Perhaps once he sins, he loses that divine power. Or, as with many mystical ideas, there is a psychological representation of it as well. Once a person “falls off the wagon”, the resulting despair makes it harder to maintain resolve. Most people think they struggle with commitments, but actually they struggle with accepting their lapse and picking themselves up and moving on. The act itself while sinful or problematic is not the major problem. Instead it’s the getting stuck in the bad place and falling further that does the real damage.