Sometimes. Not necessarily always, I don't think; and this applies to when things are too easy for the PCs as well as too hard.
No it doesn’t. Things being too easy for the characters is a very different thing.
As far as I know from the "media coverage" lo these many years, the party were utterly and completely outgunned. What I don't know are the specific details as to just how much warning they had - whether for example they saw from a distance the demons standing on the front steps and still tried to sneak past them into the tower. I also don't know how much demon knowledge the players/PCs had - this was early days and they were as yet not at a level where demons would be expected, so it's possible they didn't recognize the demons (either in or out of character) for what they were. I'm fairly sure they had no idea there was a lich in the tower (but could be wrong on that) though the lich's name was, I believe, already familiar to them kind of in a mentor's mentor's mentor way.
Yeah, there are too many unknowns to say for sure… but the gist I’m getting is that this would be problematic at my table.
And yes, the demons were rather casual about how they dealt with these annoying little PCs. I mean, they only killed three...
Who says they even need to kill any at all?
The demons were already in place as door guards.
So what? Was there a reaction roll? Anything else that was used? Again, we don’t know all the details but it sounds like the DM gave the players incomplete information and then punished them for not knowing more.
In my case, I see it as part of what the players signed up for - that fiction and character takes precedence over table concerns; and that what happens in character stays in character.
But it’s not a matter of what’s in character. It’s a question of whether a game is fair or not. I’m not saying there shouldn’t be things that are beyond the characters’ abilities to deal with… but when that’s the case, it should be communicated in a way that’s lends more context than “it’s dangerous”.
Situationally dependent. If for example at least some of the PCs were determined to take on these demons no matter what (which, knowing some of the players, might well have been the case), then what?
That’s when they’ll have made a more informed decision and so the results are on them instead of the DM.
To me this whole scenario is a fine example of character-side trial-and-error adventuring; and had I been the DM I don't think I'd have done anything differently in a broad sense (can't speak to fine-tuned specifics).
Not really… it sounds more like player side trial and error, at least for the players who lost their characters. Now they know, as players, to never know what to expect. Seems like a crappy lesson to learn, and it also seems to shatter your typical views on metagaming.
Why? If "dangerous" is all they happen to have heard then sorry, in the moment they decide to wander over there that's all the information they get. If they're not careful they'll find out by trial and error what "dangerous" means in this context when they get there.
What’s the first question when someone hears a place is dangerous?
Dangerous is typically an invitation for characters in D&D. It’s not enough context to matter.
Welcome to the sandbox.
Seems more like a litter-box to me.