Two people are defined by their relationship with one another, and one of them is killed off.
Oh, Those Two Guys, you're so nutty! We love you with your crazy background antics and hilarious hijinks! In fact, you're so close and are constantly together, you're practically The Dividual! Yup, I'm sure we'll be seeing you guys together forever. What's that? Guy #1 just died a horrifically gruesome death? Oh. ...Awkward. Where does a character who had previously been defined solely by his relationship to another character go when the other character kicks the bucket? When this happens, this usually results in one of two things. 1) The surviving character becomes more fleshed out, and is now considered their own person, instead of just one of a group of two guys. 2) The surviving character remains defined solely by their attachment to the now-deceased, and without their other half to make them whole, either sink away out of focus, or are killed off themselves.
As a Death Trope, all Spoilers will be unmarked ahead. Beware.