Hmmm... We're only talking about a movie here so...let's keep this simple

Vader's death was an enactement of sensible script
just think- It was Luke's time now- the torch passing- Anakin time was over
- Had Vader/Annakin lived how would you fit that in a script that
folks would buy into? Yeah I could really see Annakin reminiscing
about all the years-bragging about sith functions like slaying jedi
destroying planets ect with Luke-like that would go over well

- Martyrism
esque reason - because he died by turning good at the
end it made a better story
- the movie is more powerful at the end when the bad guy you
love to hate is actually a good guy after all(now you're love for
him is justified!) (my fav. reason I used to love watching Vader
choke the little bitch who got in his way or dissapointed him-
greaat!)