If you don't want to make another story following the one you're currently writing, then I suggest you end the story any way you like! I've read a few stories that have sadder ending, and they all show that no matter how great things may seem (or how lucky certain characters are) that not everything can always end well. An ending like that - or even having the death of a well known character mid book - is up to you.
It helps enhance the reader's emotions. As long as you end the story with a bit of information on how the other characters are doing (maybe as an epilogue?) after the death, or even dedicate a second book to it, to show how they're dealing with the death of Stephen. Moral; do whatever you feel is right for your story.
As long as it makes sense with what's been going on and it's too out of the blue - so as long as you have a set reason to kill your character - then do it!
It is amazing the ammount of romance stories where at least one character dies. Just watch any Hollywood romance, not romantic comedy, but a romantic drama and you will see. But generally this is for some form of catharsis but it seems you are doing for the shock value more than anything.
So unless this death serves more purpose than ruining a happy ending I would advise against using it 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.