Peanut allergy. (Just kidding.) An angel can be almost anything in your story's universe: a spirit, a creature, even a messenger who happens to be human -- the amount of magic the angel is made of should have some bearing on the amount of magic needed to take her out. It could be a cursed dagger in precisely the right spot, or an ordinary stray bullet that catches him unaware.
Nobody cares (or even thinks) that it's a 'bit dark'. We only care that you don't seem to realise that when you write a fantasy, you can do whatever you like with your characters. Kill them off however YOU think would work well in YOUR story.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.