I think it depends on the type of story, The X-wing novels had a lot of characters come and go because it would be unrealistic for a fighter squadron never to loose anyone, although they tended to be more Red shirts.

Depends on where you're taking the story. If it's necessary for character B to die to give character A incentive to go on, it makes sense. If you're killing character B to make room for more story and development for character A, you might be better off not including B at all in the story.

Let's say Villain cut a nasty gash on B during a fight. B is hospitalised, and it seems like it'll go smoothly, as B is one of our heroes. If B then dies, it gives A an incentive to further his/her battle against Villain, be it revenge or some other reason.

I agree that the death needs to make sense and have an effect on the story or the characters. A key thing too is to have an investment in the character that dies, ie, we should care enough (either negatively or postively) to feel something (and that can also happen indirectly through another character that has a strong connection to the deceased). I'd argue storytelling is entirely about playing on the emotions of the audience. When the evil witch is defenestrated by the hero in the finale, we cheer precisely because she's been so villainous up til then. When a hapless random guy is riddled with bullets on the street, we feel a tinge of sadness because we just saw him with his loving wife and he was awkwardly rushing out with the garbage.

What you don't want to do (IMO) is kill characters just so that you can prove the danger of a situation. When Redshirt 21 steps on an ancient booby trap and is blown to smithereens, you're not fooling anyone that Kirk is going to do the same.

Feel free to steal it or pass the idea on. The story I developed around that, done in the form of a diary that someone else would take over, morphed into The Waifs so I managed to get things out of my system. That book isn't first person, though

well then friend those people in particular, leave the rest behind. That's the worst kind of group.

Maybe I've taken too hard a stance on it, but it's the extremes that annoy me.
Or it's a difference of style? To me, well-developed characters are elements of the story, to be figured out as much as possible along with the plot, so there are fewer wrenches (there are always some) once the actual writing happens.
If the theme of my story is the dark side of loyalty, I had damn well better create characters who reflect that from the get go.

Or "from jump" as Teague would say

If you give characters proper motivations, then they should be driving toward whatever ending you have planned, surprising twists just adding color. If you haven't planned a good ending, you won't have a good ending, which is why most TV finales stink, even for the best shows.