The show Angel has its issues; far too many episodes are driven by one of the few female characters being captured, held hostage, or in need of rescuing in one way or another. There are no fewer than four strange pregnancies. It tends to be a tough world for the ladies while the run-of-the-mill male characters become inexplicably competent, strong, and danger-resistant.
In spite of that, I still finished and enjoyed all five seasons. (The fourth and fifth seasons turned things up a notch. Buffy and Firefly both ended, and I wonder if, because of that, Joss Whedon had a chance to give Angel more attention. Sidenote: with the Firefly's tragic demise, Nathan Fillion, Adam Baldwin, and Gina Torres were all freed up to play the supervillains on Buffy and Angel. So at least we have that.)
Whereas Buffy the Vampire Slayer ends in systematic victory (hellmouth closed and hundreds of girls around the world empowered to be slayers), Angel ends as it promises, in defeat. Over and over in the show it's established that the senior partners (the bad guys) are unbeatable. They have been around for most of time; they have offices all around the world; they deal in everything. Angel and crew even moves in to their LA office and are complicit in their schemes for a while. There will be no decisive victory over them. The fight will go on forever.
In lieu of defeating the senior partners, the crew resolves to aggravate them as much as possible. In the last episode they make a strike on the biggest players, to the powers of darkness, that are available. Angel tells them, at the top of the episode, that they are not going to make it out of this. And by the end of the episode, those characters who have not already died, are up against an army of darkness. They stand to last about ten more minutes.
It reminds me of what I've read of Norse mythology. In it, evil triumphs:
"If the gods are finally helpless before evil, men and women must be more so. The heroes and heroines of the early stories face disaster. They know that they cannot save themselves, not by any courage or endurance or great deed. Even so, they do not yield. They die resisting. [...] In the battle between good and evil they will fight on the side of the gods and die with them.
"This is the conception of life which underlies the Norse religion, as somber a conception as the mind of man has ever given birth to. The only sustaining support possible for the human spirit, the one pure unsullied good men can hope to attain, is heroism; and heroism depends on lost causes. The hero can prove what he is only by dying. The power of good is shown not by triumphantly conquering evil, but by continuing to resist evil while facing certain defeat." - Edith Hamilton, Mythology
I respect the writers for ending the show the way they did, true to the tone and premise of the show, broody and unflinching.
It's not our culture's idea of how a good story ends. And we tend to shy away from entrenched evils because, we argue, it's always been that way, and things are unlikely to change. In a world of countless problems - cycles of poverty, institutionalized inequality, pain, death, heartache, et al. - it might be fitting to take on an attitude of Norsemen and of vampires with souls.