Incendax said:
I killed a character, twice!
Every encounter is a bit of a learning experience (what works, what doesn't, what you like, what the group likes, and also for the players in terms of learning tactics and group synergy). So, my point, don't get too focused on this that it paralyzes you with over-analysis in future encounters.
Having said that, since you asked ...
Death #1 (death after regeneration): as you said, the
group misunderstood how it worked, leading to the deadly tactical choice. I can understand it and no one knew any better.
That being said, ask yourself this --
if there was someone constantly healing unconscious people back, would the enemies have gone after the healer with just as much effort as they did against the regenerator?
If no, they wouldn't have gone after a healer with the same effort, then yes you did single her out. If yes they would have gone after a healer with just as much effort, then you did not single her out and you played the monster consistently with how you envisioned their intelligence and tactics.
(Just wondering, how smart (and/or organized) are those enemies that did this?)
Death #2: beating someone to unconsciousness and then "throwing them down
to be with the rest of the party" that seems excessive -- why would they throw her down with the rest of the party?
If they threw her down because that is the feeding pit for some creature they worship, that's another story. But if the justification for them throwing her was to put her with the others just seems odd (to me) - given that height and no foreknowledge of the party, the monster may think that the party members that fell actually died.
If they wanted to eat her, they should have eaten her. If they really just wanted to be left alone, then after everyone else was gone and she was surrounded they may have said "leave now and never return" (indicating that she can climb down on her own, possibly after demanding some gold/jewelry/magic item she has on her).
If they wanted to bring strife/something to the nearby town, they could have planted some hypnotic suggestion in her and then let her leave only to act like a 'sleeper agent' in town (to do something that the creatures could not do themselves or was too dangerous) adding a new plot for the party to deal with snapping her out of it and undoing whatever damage was caused.
Anyway, my point on this death is, I think there should have been a point when it was an obvious victory and asked yourself "what really is the motivation of these enemies (food, defense, privacy, etc) and is there a way to turn this into a cool (yet natural and believable) plot hook?" and then just worked from there.
Hindsight is 20/20. In the heat of the moment, with all the other DM-related things going on in your head, it's easy to get caught up and do something that you second guess. But give yourself credit for a) DMing which is often a thankless job in itself, b) getting regular feedback from the players, and c) trying to find ways to improve future encounters.
As a personal anecdote: in my time DMing (even some recently) there have been PC-kill moments that I questioned and regretted after the fact. But you just learn from it and go on.