1. How would you feel if your favorite character died?
I would be upset, of course, but whether I'm mad at the writer just depends on how it's done. If it furthers the plot in a way that makes sense, I don't mind as much. I also think how the characters in-story react to the death. Sometimes a character will die in a story and the surrounding characters will go "damn that sucks" and move on, which just isn't realistic and leaves me kind of lost. This is why death for shock value doesn't work out, because much of the time the characters don't stay shocked or sad for very long.
For example, in the Harry Potter series (Order of the Phoenix and Deathly Hallows spoilers ahead),
Spoiler - click to show.
I didn't have a problem with Sirius dying even though he was one of my favorite characters, but was it really necessary to kill off nearly every side character at the end of Deathly Hallows? Sirius' death made sense, and it helped further the plot through how it affected Harry, but the scores of deaths at the end of the series just felt like it was for shock. It would be weird if people didn't die at all at the climax, but there were a lot of deaths all at once of characters I genuinely liked without any time to really process it.
My favorite characters are often villains, too, which can be a whole other thing. Sometimes the only way that makes sense to resolve an arc is to kill the villain, so I'm usually a lot more okay with death if the character in question is an antagonist.
2. How would you feel if the main character (not necessarily your favorite) was killed?
This one really depends on whether it makes sense for the plot, and whether it'd be a satisfying conclusion to that character's arc. Sometimes the only way to resolve something in a story is for the main character to die, usually by willingly sacrificing themself, and it wouldn't feel right if they magically survived. It's a lot more likely to be disappointing if, say, the character actually makes it out of whatever impossible thing alive only to be killed right after. The book Allegiant, like @Mockingjyn mentioned, is a good example. It feels like you got invested in the character's story for nothing.
3. How would you feel if a character just disappeared without any evidence of their death, but would be considered dead anyway?
It's fine if the reason for their disappearance/death isn't revealed immediately, but I'd want to get confirmation on what actually happened at some point.
4. After reading a book, how do you know that there were "not enough deaths"?
Really depends on the genre. I've been shocked by the amount of deaths in some seemingly lighthearted stories, but if I'm reading, say, a dystopian novel like The Hunger Games, it would be really strange if hardly anyone died. How much suspension of belief you can rely on is dependent on the genre. If no one died in a more lighthearted fantasy story, even if the stakes were pretty high, I'd be willing to wave it off, because that's not really the genre. But if it's a really violent story and everyone just miraculously survives, it becomes a little too unbelievable, and takes out some of the immersion because it just doesn't feel right. That being said, again, it's up to the genre. Not every story needs a high body count, or even any deaths at all. I have a hard time killing off characters myself, so the plots of my stories are generally light, and the heaviness comes more from emotional conflict, not violence in the plot. This one is completely up to the story in question.
All in all, I'd say it really just depends on the execution. I also don't think there's much of a point in killing off characters just for shock value; there really needs to be more of a reason behind it. A character death doesn't always have to directly further the plot–there's a lot of stories that wouldn't be as believable without a higher body count–but there needs to be some sort of justification. I'm kind of a wimp when it comes to character deaths, so maybe I'm not the best person to be getting an opinion from- but I really just get disappointed with the author if character deaths are done poorly and come from completely out of nowhere, seemingly just for the hell of it.
