There are a number of reasons why people kill off characters, but only a few that make a good story. They have to help the storyline in some way shape or form. To get you from one stage of the book to the next, or story complexity and character development.
Some authors and writers will say killing a character just for the sake of shock and awe isn't smart, but I mean sometimes you're the story can be rather flat. I think it all depends on the story- it's a case by case basis. I have killed off a character that didn't necessarily need to be killed, but it strongly helped the development of my characters and chapped her answer viewpoint for further confrontations. That is important.
Make the death mean something, or it isn't necessary and readers will point it out.
~Get yourself into the mindset
This is just as important, if not more so then writing the actual scene. You have to get yourself set up for the scene and make sure you are in the mindset which will take you to a place of that kind of harsh and raw emotions. There is a group called Sleeping at Last, which have amazing music I used every time I write a death scene/ sad scene. The song Hearing- it alone makes me cry like a baby. Letting yourself get into that vulnerable state help make it all real- and that's exactly what you want as a writer. The story is made up but the emotions are real and that's what make's a beautiful book.
I also have a few people I know who watch youtube clips of there favorite death scenes, to help pull some good inspiration off of those. This might help if you aren't quite sure how to go about it and need a visual.
~ Make them loved- or make them important
The person your killing has to be someone people have extreme emotion for- they either love them or hate them. Making sure they are extremely loved or cared for it a little easier and it will make it more heartbreaking when they are taken away. If they are a static and flat character- the emotion you want to portray to the readers might be more difficult then it's worth.
~ Unfulfilling their purpose
behind every character developed there has to be a reason why they are there. What is their purpose? Why are they apart of this? If they don't have one, then they're death won't mean much to your reader.
~ Physicalities of death are important (Little rant)
This one always bothers me given I also work in the medical field. Make sure you really think about how your character is dying and try to give me details that are obvious or worse, don't make sense at all!
If they are shot in the neck, they literally have 30 seconds before they bleed out and they won't be able to talk, so please don't give them a speech.
Please do the research.
Did they hit a vein, then it's most likely kind of run out at a quick passe, and they might not go as fast as an arterial bleed, but it's a matter minutes before the person passes out from the blood loss. If it's an arterial bleed, it's literally shooting out and will literally take less then a minute before they die unless you can get it pinched off.
Look up signs of shock.
Think about how someone would react to not being able to breathe, or how a poison would react in someone's body, they might have a seizure. Just take a practical standpoint sometimes on things like this, it will make it sound real, help with creating a more obvious death.
~ Reactions (MC death vs. SC death)
*MC** If death is happening to your character in which you are speaking first person through, the clock is going a little fast for you. You are dying, and there is only so much you can say and feel based on your injuries. Panic is a big thing for people when it comes to there own, I mean I can handle a lot of blood, but when it's mine- I panic. Sometimes it might take a while before the person begins to panic, and there are two routes of panic; fear or anger. Based on your character's personality you'll have to figure out which path they would choose. People sweat when they are losing a lot of blood, they go very pale because the body is working harder to move the blood around the body and is taking it away from less vital organs, like your skin, to aid organs like your heart, lungs, brain, etc. They get headaches, their fingers and toes begin to tingle as the blood is shunted away from their limbs, again to the most vital organs as your body is fighting to keep the body alive as long as possible. Their vision might get spotty or they might not be able to see at all. Fear is a big play toward the end, but as the end comes, people are less able to more or feel things.
If your character isn't dying from bleeding out, your character might not be able to know what's going on. If they were poisoned they might not be able to breathe, or they might have excruciating pain and then everything goes black. if they have a brain hemorrhage they could have an intense headache and then pass out. Heart attack, people get nauseous and dizzy, they have the chest pain, or neck pain, or arm pain, but they also sweat profusely.
DO YOUR RESEARCH.
*SC** If it's a character your first-person character is reacting to, you have more time to go through the processes of the death since it isn't happening to you. Based on your character's experience with death and dying, your character is going to panic, it's what they do with the panic that's different. Will they try to help? Will they sit frozen or cry? People who are watching someone dye tend to repeat the same words over and over. "You're going to be okay, stay awake, don't do this, etc." Depending on if your character wants the person to die or not is a whole other thing, right now we are just focusing on them not wanting them to die." You really have to live within the moment, don't have your character thinking of memories or recalling moments together, that's for after.
There is panic, anger, fear, and the unknown and you really have to play on those emotions. Really feel them.
~ The aftermath
Depending on the death the character might have to act right away and feel emotions later. War zone, attacks, or natural disasters can be reasons why your character can't morn at the moment right after it happens. When your character mourns, there are a number of the way people do it, but it's very amped up. If they are angry, they are raging and screaming, make sure to make it intense. If they are sad, they are hyperventilating, they are vomiting, they are in full range panic. If they are still in shock they can be in a daze, or staring off at something without any sense of time. If they are ignoring it all together they might act erratic and constantly be on the move without sleep. The thought of death is very bewildering and people take it differently but it's always extreme.
This is where they can recall memories or questions. This is when they can say 'if so and so we're here ... ' This is when you can begin the stages of grief, make sure to look them up.
~If you don't feel it, your readers won't
If you want to make your readers made about the death, they by the end of it you have to be angry! Enraged! You have to feel that emotion in order for your readers to feel it. Same goes for feeling sad- you want your readers to cry? You want them to ball their eyes out? You have to end up crying by the end of the scene honestly!
I hope some of this has helped if you have questions feel free to message me about it! Questions about different types of death- I will be more than happy to help! Happy Writing!