The Kill 'em All ending was such a ridiculous Tear Jerker for its earliest audiences that a different, happier ending was created for productions in later centuries, one that lasted until the 1900s. It wasn't until after World War II that the original ending was performed again, and once again the tears began to flow.
Also, this (slightly earlier) exchange:
Lear:If you have poison for me, I will drink it. I know you do not love me; for your sisters Have, as I do remember, done me wrong: You have some cause, they have not. Cordelia: No cause, no cause.
Why should a dog, a horse, a rat, have life, And thou no breath at all?
I have a journey, sir, shortly to go. My master calls me; I must not say no.
A mention also needs to go to Lear's breakdown before Act III, when Lear discovers his daughters conspiring against him. Especially when King Lear basically tells them "even beggars have things they don't need".
The scene where they do the fake trial against Goneril and Regan (which sadly, is often cut out of productions), and Edgar's reaction to it. Despite all the crap Edgar's been through (his half-brother tricking everyone into thinking that Edgar's a villain, forcing him to flee his beloved father and home and pretend to be a madman just to stay alive), he still feels nothing but sympathy and pain for Lear, ignoring his own pain.
The production with Derek Jacobi: the first time at the end when you hear him "HOWL!"
Sir Ian McKellen as Lear, every time he's onstage from the storm scene on.
After the ending, the few characters that were still alive began to "raise" the rest from the dead, touching them one by one, ending with Regan and Goneril who went to Lear, and Lear who went to Cordelia.