A phrase you use when you're busy killing somebody but they're taking way too long to actually die and you're getting impatient. Alternatively, it can be directed towards someone who is alive and in no danger but you hate them intensely and can't wait for them to be killed somehow.
Seriously, how is this guy still breathing? Just die already!
No seriously. This is a reference to Breaking Bad season 1 episode 4, when Walter Junior, upset and in turmoil over his father's lung cancer diagnosis, outrages at his farther's hopelessness. His mother, Skyler White, is visibly distraught and in shock. Walter White, his father, is facially ambiguous, but I suggest that he is turned on by the unexpected lash out. The abruptness and out-of-pocketness makes this one of the funniest lines in Breaking Bad, nay, in American television.
The writers could well have ended the entire TV show by having Walter White die right then and there and rolling the credits. In fact, this would have been a totally viable ending to a long-winded story and would have prevented the rise of the crack-addicted, mentally unstable Breaking Bad community. Just imagine a world free from crack-heads yelling "JESSE WAKE UP" in public.
The phrase is most often unintentionally used by rage-quitting gamers; specifically, Minecraft zealots when they have two hearts, no shield, and three skeletons are raping them. The phrase also features in many political debates when the opposition runs out of rebuttals and resorts to crude insults, because, after all, this is the best way to assert yourself in discourse.
*3000-word assignment exists*
Professor: "You already have a lenient 5-day period to complete the short-release assignment. However, given the astounding number of simple extensions, we have decided to provide a 2-day blanket extension."
Students: "Why don't you just fucking die already?"
You: *empty a Vietnamese War amount of bug spray on a cockroach*
Cockroach: *moves*
You: "Why don't you just fucking die already?"
Osama bin Laden: *becomes the most elusive terrorist leader in history*
Americans: "Why don't you just fucking die already?"
Stranger approaching you with a clipboard and optimism: "How do you feel ab-"
You: "Why don't you just fucking die already?"
*waiter places your food in front of you*
Waiter: "Enjoy!"
You: *accidentally* "You too."
Waiter: *eats your food*
You: "Why don't you just fucking die already?"