Honestly, I think I'd rather see a beloved character die than become corrupted by evil. I mean if the journey is Good Guy=>Fall From Grace=>Evil=>Redemption sure. But to end the story with "Mwah-ha-ha, now my death ray will destroy Metropolis!" is a much bigger let-down than having the character die.
I think the biggest, most important, thing is for the end to fit the character. There are some characters who by the end are so thoroughly broken (see Wesley on Angel) that as sad as it is, their death is in some ways a mercy. There also times when self-sacrifice is necessary.
As long as the death has a meaning or purpose, I'm fine with it. Hero saves the day only to get hit by a bus, not so cool.