If I am hiking in bear country, I am usually aware that there is a chance of meeting a bear.
Except, with random encounters, you've typically got a large list of possibilities. If you enter a region infested with undead, sure, you might know that you will run into undead. Does that mean you will realize that you will run into a Bodak? Does the DM instead drop signs of every single possible creature they might run into? Does the party have a cleric who can recognize some of those signs belong to a Bodak? What if he fails his check?
And once the players know, what preparations are they going to take? Sure, if they have a cleric, and he is high-enough level, he can prep the right spells to save they day. What if they don't have one? Just keep their eyes closed all day to ensure they don't see a Bodak?
And this doesn't help with the presence of Save or Die spells. I asked you before a few questions regarding a situation with a wizard, and how you might handle it, but didn't see any answers.
Part of the problem I'm seeing with some of these answers, such as "be prepared for anything you could possibly fight" or "always carry antitoxin" or the like, is that we start to get into an arms-race between the DM and the players.
If a Save or Die effect exists as part of a rare and unique encounter that the party can prepare for, they can figure out creative solutions that get around it.
If these effects are prevalent throughout the game, it instead becomes a battle of system mastery. Are the players aware of all the possible dangers that can end them in one shot? Have they loaded up their characters with a very specific list of spells to counter certain dangers? Have they invested in the most ideal feats and items to give them rerolls and immunities and bonuses?
And sure, that is one style of play. But it isn't necessarily for everyone.
AFAICT, there has never been a system for choosing monsters that doesn't ultimately rely upon the common sense of the GM, and fall down if said common sense is lacking.
Again, putting too much burden on the DM. Saying that the DM is as fault for using a monster or spell as it is presented in the rules means the problem is with the game, not the person running with it.
Assassins randomly targeting people walking on the street is poor DMing. Usually, if there is an assassin after the PCs, there is a reason for it. And usually the PCs are aware of the reason.
The PCs might know that they have earned the emnity of King Spiteful. Does that mean they know exactly what form of retribution he might take?
Ok, perhaps they do - perhaps they have spent time researching it and asking questions so as to be prepared (and hopefully don't have other quests distracting them from doing so). What if an assassin wasn't his first choice? What if he instead likes to consort with warlocks and demons, and the PCs figure this out, and thus demolish every summoned monster he sends against them. What if eventually he diverts from his usual approach and hires an assassin, hoping it will be more successful?
Are the PCs at fault because their enemy was creative?
Again, you seem to be saying that in your games, the players never fight an enemy without knowing exactly what it is capable of, well in advance. I can see that style working, sure, but I don't think that is the default for the game. Most games tend to have it go both ways - some fights the players might be well-prepared for, others might have surprises.
I am curious where you get "the purpose of the SoD effects, as presented in the rules themselves.....is to kill the PCs" from, though. Can you quote that?
"In most cases, a death attack allows the victim a Fortitude save to avoid the affect, but if the save fails, the character dies instantly."
It is a mechanic that causes PCs to die. That is what it does. If a medusa had a special section with detailed rules on reflecting its gaze back upon itself, and ideas for helping PCs prepare for such a thing, I might feel there was more to it than that.
But as it is, the effects of these abilities are to kill PCs. For someone reading the rules and running the game as it presents itself, there is no reason to assume that the goal of a Finger of Death spell is to encourage all PCs to walk around with a bag filled with Death Ward bucklers. No, the purpose of Finger of Death is to show that this wizard is a complete badass, and he can kill someone by pointing at them!
Which is an awesome image, sure, and I can see why people like having that sort of thing in the game. I can also see why others don't.
And I still don't buy the argument that Save or Die effects are fine because the game assumes that PCs will never actually run into them and risk dying unless either the players have screwed up, or they have a bad DM.
Save or Die effects are in the rules because the game assumes that, sometimes, PCs will have to Save, or they will Die!