Beni: But I'm afraid more is needed. The prince must finish the job.
When Horus (Ardeth's falcon) was shot out of the sky.
Hell, the death of all the Americans in the first film count. Burns died alone, blind and terrified, with no one to turn to for help or comfort. Henderson, you might say the warrior of the three, didn't even get a chance to face his attacker and go down fighting. While poor Daniels was backed into a corner and was unceremoniously murdered while trying to pacify his attacker. We should just consider ourselves lucky we only heard what happened to the egyptologist and weren't forced to watch...
Imhotep losing the love of his life at the end of the second film, when Anck-su-namun abandons him to die. He sacrificed everything for her: his life, his soul, and it was all for nothing. The look on his face when he realizes this, and the wistful smile he throws Rick and Evelyn, is heartbreaking. Despite everything he's done, it's possibly one of the most effective cases of Alas, Poor Villain in recent memory.
The novelization states that Rick himself feels so sorry for Imhotep, he actually tries to help save him... but Imhotep still lets go and falls into the underworld.