Good characters don't usually die at the end of an Elizabethan play. The idea of the tragedy is that a person of noble rank (in this case a king) falls due to some fundamental flaw in his character. During the play, any character who commits a sinful act will die, in order to restore the natural order of being. This can be said even for the character of Cordelia, if you take the view that she has an obligation as a daughter to obey her father, by refusing to "mend her speech a little" she is ignoring this obligation, and therefore commiting a crime against the Order.
If however you take the view that Cordelia is virtue personified, you can say that she was killed as a punishment to Lear for his own sins.
I wouldn't recommend talking about the production where the ending was changed, because that isn't the version you are studying. you can say that this did happen, but it cannot be the only version you respond to. you must respond to the version you are studying, in this case the script you are reading.