I feel like I keep stumbling across stories about this over the past week. RPG Horror stories that frequent the theme of the GM or DM planning on, and in some cases outright saying, that they were going to kill at least one PC in a given scenario or boss fight. If I'm being charitable I can kind of get the idea. They want a boss to be scary. They want it to stand out. They want it to be a fight that the PCs remember or that meets their definition of 'adequate' amounts of cool for the boss. And that means not being a chump who gets wrecked without even taking someone with them.
The problem is that if you have in your notes, or your head, that you're killing a PC you're already doing it wrong. And if you tell the players you are doing this, you've already ruined the encounter - and likely their trust in you as a GM.
PC death should come from three places. First, the dice themselves. If bad dice rolls add up to a player dying, while unfortunate it is part of the risk of combat and RPGs. Second, a player made mistake or choice. If the player's own actions lead to their death, then that is on them. Third, at a player's request. Sometimes a player wants a character to die for some reason - how they see the story ending or to bring in someone new. If a player asks for it and you work it out, hey, no problem.
PC death as a deliberate part of the GM's plan though? That's just being a dick. But let's break it down.
From the GM Side
Your boss doesn't become cooler, more powerful, or more memorable if it is hard written or planned for them to be able to kill a player. To be clear, there is a big difference between "this is a challenging fight and could result in some deaths if the dice go wrong or the PCs play it wrong" and what I am talking about. One is making a challenging and dangerous encounter. The other is an execution.
And it is an execution because as the GM you can control this. You can force the outcome. You can cheat the dice rolls. You don't even have to roll to attack, or for the bad guy to make saves. There's no drama in the boss killing a PC, because it was never in question. It's not a testament to how cool or powerful they are, because you are fiating the death instead of using the boss well or in a fashion where they can 'earn' that themselves so to speak.
Finally, you as the GM should never be planning to kill the characters. Even if you think it would make the story better. Even if you think it would add tension or gravitas to a story. Unless the player has asked for help killing their character, you shouldn't be planning it. NPCs can, and you can make threatening encounters. But not "and I'm going to kill at least 2 of them before XYZ happens."
From the Player Side
Let's say you do this. Let's say you plan to kill a PC. Even if you don't tell the players they're going to pick up on it. It's easy to tell when a fight is meant to kill someone and when a fight is meant to be beaten - especially from a GM who occasionally just decides to kill a PC. And when it is done to sell an NPC it becomes more obvious because quite frequently there are tell tale signs that this NPC can't be beaten - at least not yet - and is going to be coming for more.
So what does that mean on the player side? Well, for one it kills investment in the game. Why should a player get invested in making a character, exploring a character, and trying to grow a character if at some point the GM is just going to arbitrarily decide to kill them? Why invest at all in that situation?
For another, with trust broken, it means even when things happen legitimately they'll be suspect. You can't trust the GM to run it straight. You can't trust you have a fair shot or your ideas will be given a fair chance unless they're what the GM planned for. So why bother?
And for third, with the above two happening quite frequently the third part is "this game is no longer fun for me, so I'm out."
I've been a player in a game where only things the GM wanted to happen happened. And quite literally no one cared about what happened. We didn't quit, but we also didn't really help. We just went along with things and there was no gravitas or drama. Big moments fell flat. We didn't care about NPCs. We didn't care who we helped or didn't help. The game continued - it was something to do and several people had to be at the location at the time anyhow - but it never got better. And after? No one has agreed to play with that GM again because it just...felt bad.
Other groups may like that. And that's fine (hooray for Session 0). But it wasn't for us, and ruined what could have otherwise been a fun game.