Not Rashi by Kayin but the pasuk itself tells how good and bad behavior are dealt with, and your Rashi above, even though not mentioned, has to the refer to the opening in the hey otherwise a ches would also suffice. The GRA explains that on Pesach chametz is assur bemashehu because the difference between chametz, the yetzer horah, and matzoh, the yetzer tov is only that mashehu, the opening on the hey. If we don’t watch matzoh carefully, it becomes chametz. That is what we say Ha Lachmo Anya, the hey makes it a lachma anya.
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