Musar for Berakhot 72:19

This is the reason that Cain, who represents the קליפה, was born before his brother Abel, who represents the fruit. The former said to G–d (Genesis 4,9) after having murdered Abel: "Is it my function to serve as the קליפה, "husk," for my brother?" Our sages understand this comment as Cain referring to this function in his life, because they understand the meaning of the word שומר to be the same as שומר הפרי, "the protective shell around the fruit." The letter ה in front of the word השומר indicates Cain's ongoing amazement. He cannot get over the fact that the evil urge within him, i.e. the קליפה, so overpowered him. His reaction was similar to ours when we acknowledge having sinned, blaming the evil urge within us.

In all matters of disputes between the school of Hillel and the school of Shammai we rule according to the view expressed by the school of Hillel, except in six instances. This means that the views of the school of Shammai are ignored when it comes to practice. Why, in this instance, does the Talmud appear to give one the choice as to how to conduct oneself?