Be careful with your characters

With the recent death of my father, I have picked up many of his old books. One series I looked across for the first time in years was Peter O'Donnell's Modesty Blaise.
Formerly a newspaper cartoon, then a series of books, they all have the same basic plot: Bunch of evil-doers run into Modesty and henchman Willie Garvin, who get captured, then have to use their skills and know-how to escape and defeat the baddies. It's good, clean escapist fun.
There was one new book I hadn't read before - The Cobra Trap - a series of short stories involving Blaise and Garvin. To my dismay, I discovered the final one (spoiler alert) killed them off.
Now this is a pair who had cheated death in so many ways over the years. But the way they were killed off truly turned my stomach.
There seemed, to me at least, to be no point to it, beyond finishing the characters' stories.
Whether you have read these books or not, whether you were a fan or not, to treat a major character with so much disdain really didn't sit well with me. You have these two characters, which O'Donnell had been writing about for 50 years and they were thrown away in less than 100 pages.
If I was one of those characters, I wouldn't be happy.
As an author, I feel deeply responsible for my characters. They al live with me for years before escaping onto paper.
As a reader, I feel cheated that characters should have so poor an ending.
I guess it's just another message to authors that, ignoring the George RR Martin factor, you should always be mindful of how and why your characters die.