9. Charlotte's Death
Charlotte Richards, like most of the characters in Lucifer, can't necessarily be described as a good or bad person. She had committed her fair share of sins, but, also just like the other characters, was trying to be better.
In the season three episode "Quintessential Deckerstar" Charlotte takes a bullet for Amenadiel after Cain attempts to kill him and frame Lucifer for it. As she dies in Amenadiel's arms, the former angel gets his wings back and flies her to the gates of Heaven.
While her selfless sacrifice redeemed her of her wrongdoings, the tragedy of it is that she still had so much potential for growth. If she had stuck around, who knows how far she could have come by the end of the show.
Her death was sad enough, but it's the aftershock that helps seal the deal here. Charlotte's death is felt throughout the entire rest of the series. It affects just about everyone, but it especially Dan.
Dan is quite literally never the same after this. His entire character arc is influenced by his grief and guilt over Charlotte's death. Her dying fundamentally alters part of the narrative's direction up until even the last season. When you think about it, that's pretty darn crazy.