This is found in several other likutim written by Rishonim on Chumash, namely, the Imrei No'am of Rav Yaakov D'Illescas (שמא היה מתפלל על בעלה שימות וזה לא היה טוב) and the Moshav Zkeinim (ושמא נתן עיניו באשת איש בשעת אזכרה שימות בעלה), both of which were written in the fourteenth century and both of which gathered Torah from the Ba'alei Tosfos and other Rishonim. I quoted Rabbeinu Yeshaya's sefer because he is the only one who identifies the source of this idea as Reb Yehuda Hachasid, while the Moshav Zkeinim only says "Hachasid." (I've seen several people who erred as to who "the Chasid" was.)
Basically this is what he says: A kohen gadol may not marry a widowed woman. Our passuk, in 21:14, says אלמנה וגרושה וחללה זונה את אלה לא ייקח כי אם בתולה מעמיו ייקח אישה. Why is a widow prohibited to him, when she is allowed to other kohanim? Reb Yehuda Hachasid explained that the Kohen Gadol has great power when he enunciates the name of Hashem on Yom Kippur, and perhaps he has his eye on a married woman and he will use his power to cause her husband's death so that she would become available to him.
This is more than a little odd. Yes, we know that something along those lines happened to one of our kings, a man beloved by the Ribono shel Olam, a holy and spiritual man. But for Reb Yehuda Hachasid to say that the Kohen Gadol, the most holy of the holy Kohanim, on the holiest day of the year, while standing on the holiest spot on Earth, in front of the Aron Kodesh, enunciating the holiest secret Name of Hashem, (a Name that most humans could not survive saying and when others hear it they fall down on their knees and pray,) and considering that the Medrash in Acharei Mos (21:12) says that the Kohen Gadol was not considered to be a human being at that moment, he was, for all intents and purposes, an angel: וכל אדם לא יהיה באוהל מועד בבואו לכפר בקודש (ויקרא טז,יז) אמר ר' אבהו וכהן גדול לא אדם היה? אלא כההוא דאמר ר' פנחס בשעה שהיה רוח הקודש שרוי עליו היו פניו בוערות כלפידים עליו הדא היא דכתיב (מלאכי ב:ז) "כי שפתי כהן ישמרו דעת ותורה יבקשו מפיהו כי מלאך ה' צבאות הוא'" and he's got his neighbor's wife on his mind, and he might pray that his neighbor should go to his eternal reward so that the Kohen Gadol can take his widow???? Are we talking about a mee'isseh sheigetz or an Malach????
This is passing strange.
And even leaving aside how strange this is, what mussar haskeil do we derive from it? Wouldn't we be better off not hearing this depressing dvar Torah? Why would these rishonim write it in their sefarim for us to see?
The answer is, no, it's not strange and it's not depressing, and yes, it is important for us to know. It is comforting. We are subject to the power of the Yetzer Hara until the day we die. There are moments of weakness and we might have a fleeting thought that is completely at odds with who we are and what we've made of ourselves. Such moments are not a reason to despair and to think that we are fooling ourselves into thinking we're decent people. We are good and we are holy people, and when we fail or lapse, the challenge is to overcome these moments. We can't ever be sure they will never happen; we have to be on guard at all times, and when they break through, we can strive to overcome them.
As the Gemara in Shabbos (88b] says, the Ribono shel Olam wants people who were on the forty ninth level of Tumah, and who seven weeks later stood at Har Sinai ready to receive the Torah. The Kotzker Rebbe said, the Ribono shel Olam says ואנשי קודש תהיו לי. I have enough Malachim. I want Men that are holy. It is that willpower to overcome, the courage to never despair, the drive to change ourselves, the strength to fight the battle, that is what is precious to the Ribono shel Olam.
The anti-religious in Warsaw once put on a play, in which they re-enacted the procedure of ensuring that only those without sin would fight in our wars. The conscripts were asked, did you do this? And half left. Did you do that? And more left. As they got down to more and more minor sins, more and more left, until there were only two soldiers left, two bent and bearded talmidei chachamim. And then they were told that if they found a Yefas Toar, they were allowed to take her on the battlefield and bring her home as a captured wife, all because it was assumed they couldn't fight their yetzer hara. This, of course, was the big joke of the evening, and all the enemies of Orthodoxy had a fine time. But it's not a joke, and the Torah understands that there is regular daily behavior and then there is anomalous behavior and behavior under conditions of great stress. We don't sweep it under the rug: we admit the possibility of sin and we prepare for it and we learn how to deal with it.