I'd like to see alignments replaced by allegiances, but that's mainly because I'm such a d20 Modern fanboy. 
Allegiances allow you to differentiate between a guard who knows he's working for the evil warlock and is loyal and dutiful but doesn't enjoy hurting people or go out of his way to inflict pain and a guard who happily and sadistically takes care of the warlock's most cruel wishes and takes evil actions of his own initiative. The former has an allegiance to the warlock, while the latter has an allegiance to evil (and possibly the warlock as well).
It's a bit more realistic than a keep entirely full of evil people, which is what we end up with most of the time in the games I've seen.
That said, I didn't have a problem with alignment in my D&D game, and I was even mucking around with it -- I had deities whose alignment varied from nation to nation as they were perceived (and faithfully worshipped) in different manners. Example: A water deity might be a merciful, life-granting Neutral Good in the desert but a cruel and capricious Chaotic Evil in the stormy, windswept northern islands.

Allegiances allow you to differentiate between a guard who knows he's working for the evil warlock and is loyal and dutiful but doesn't enjoy hurting people or go out of his way to inflict pain and a guard who happily and sadistically takes care of the warlock's most cruel wishes and takes evil actions of his own initiative. The former has an allegiance to the warlock, while the latter has an allegiance to evil (and possibly the warlock as well).
It's a bit more realistic than a keep entirely full of evil people, which is what we end up with most of the time in the games I've seen.

That said, I didn't have a problem with alignment in my D&D game, and I was even mucking around with it -- I had deities whose alignment varied from nation to nation as they were perceived (and faithfully worshipped) in different manners. Example: A water deity might be a merciful, life-granting Neutral Good in the desert but a cruel and capricious Chaotic Evil in the stormy, windswept northern islands.