I've seen things you people wouldn't believe...
Often, the antagonist is much more interesting than the protagonist. Roy Batty, leader of the rebel replicants in Blade Runner, is a good example. Beyond one of the most famous monologues in movie history, Rutger Hauer managed to be Roy Batty. Few actors have delivered such an efficient portrayal of an artificial being, and probably some have taken it as an inspiration for their work.
Roy is the 'leader' of the outlaw replicants, who takes them to Earth to meet their maker, Tyrell, looking for a longer lifespan. Roy himself is the summum of the replicants, more intelligent than its creators.
The light that burns twice as bright burns half as long... and you have burned so very, very brightly, Roy
But Batty is the fallen angel. He betrays and kills Tyrell. He sees his comrades die. And when death comes to him, he finally loves life: just as he is dying, he saves its hunter from death.
That was irrational of you. Not to mention unsportsmanlike
Further, Roy Batty has character, he has charm. Just like the great HAL, he manages to be the most 'human' in the movie. His death is also moving.
source: the Blade Runner faq