I’m one of those who plans everything from the start, including character death. Personally, I think you shouldn’t kill a character unless it’s absolutely necessary for the story, like to bring on a new plot point, to facilitate character development or (particularly for action series) to make the story feel like the characters’ decisions actually have consequences. (I did this last one with a story I’m currently working on, where with the setting it has going on it wouldn’t be believable if every character just miraculously survived somehow.)
If you do decide to kill a character off, I think it’s better to do it after providing sufficient info about their character, their backstory, etc. It’s very unsatisfying to see your favorite character go after only getting like two panels of flashback about their actual backstory.
I actually only kill my characters very rarely. What I usually do instead is simply hurt them relatively badly, which creates both tension and character development (since, you know, they could theoretically still die, but no one except for me knows).