Beis HaMikdosh and Shechinah

Question

I heard some mention the Gemara in Gittin which says that Titus y"m went into the קודש הקדשים with a harlot. He slashed the curtains, opened a sefer torah and had relations with the harlot. The Jews knew that if the kohein gadol messed up on the avodah in the קודש הקדשים he died on the spot, yet Titus did not die, but emerged from the קודש הקדשים unscathed. The reason Titus did not die on the spot was because there was no kedushah (i.e. the Shechinah departed) from the Beis HaMikdosh. Once there is no Shechinah remaining in the Beis HaMikdosh it just a bunch of bricks.

1. Where in the Torah does it says that if the Kohein Gadol messed up on the avodah in the קודש הקדשים, he died on the spot?

2. Is there any commentator that talks about the Shechinah departing from the Beis HaMikdosh?

Answer 1

To your question about Kohein Gadol messing up on the avodah and dying on the spot, there is a gemoro in Yoma 9a

“The Second Temple stood for 420 years, ve-shimshu bo yoter mi-shelosh meot Kohanim, and more than 300 served as High Priest over that period. Take out forty years in which Shimon the Tzaddik was the High Priest and deduct another 80 years in which Yochanan the Kohen Gadol served, and ten more years that Elazar b. Charson served.” That leaves 300 priests over 290 years. “Kol echad ve-echad lo hotzi shenato, none of them lived out their year in office.”

Rabbi Shmuel Herzfeld says

When he entered the Holy of Holies, he literally put his life on the line. If he made a mistake in the service, if he had a bad thought, or if he was simply unworthy, he could instantly be struck dead from heaven.

This, he explains, is why the Mishna in Yoma 70a says

ויום טוב היה עושה לאוהביו בשעה שיצא בשלום מן הקודש: “The High Priest would make a party for all his loved ones when he entered the Sanctuary in peace and left in peace without injury.”

About the second question:

See this article

which says what I have learnt

The Talmud teaches that the Beis HaMikdash was destroyed because of baseless hatred. Bnei Yisrael did not properly fulfill the mitzvah to “love your fellow as yourself.” Their strife and infighting caused the Shechinah to depart from the Beis HaMikdash, and once that happened, our enemies were successful.

The source is Sanhedrin 96b

A divine voice came forth from Heaven saying to Nevuzaradan "Thou hast slain a dead people, thou hast burned a Temple already burned, thou hast ground flour already ground.”

which means that the essence of the Beis HaMikdash was already destroyed when he destroyed it because the Shechinah had already departed.