Hiya!
This is one "reason" why I don't "Design Encounters for the PC's Capabilities". This ties into it directly; if I "custom make" an encounter to specifically avoid 'picking on' some race/class that the PC's don't poses (or are weak at/in), and then, through bad luck and/or bad decisions, one of the PC's "goes down"...well, as DM I am now VERY much in the position of "If the downed PC gets attacks...it, quite literally,
is my fault.
I am
specifically 'choosing to have the monster kill the PC'. Me. MY choices".
BUT... if I don't care what the PC's capabilities are or aren't, past the most basic of bases (re: "X number of PC's, about Y average level"), and I "Build for the Campaign World"...then the Players know and understand how the 'world' works; it doesn't care about them. It isn't there for them to "get it" and "win" (...I always flash back to the interaction between Jack Burton and Lo Pan when I say that... "...Mr.Burton, you are not brought upon this world to 'get it'..."

).
So, if the Players know that the monsters/NPC's will react according to their own internal consistency that has been developed over months or years (or decades, in our groups case, all things considered)...then they know that when an NPC/Monster "attacks a downed PC", they aren't surprised.
Typical examples: Goblins will move on to the next target...Zombies or Ghouls will drop down and start eating. A Dire Bear may stand over and try and 'frighten' away the PC's, or try and drag the PC off. A group of Orcs may or may not try a Coup de Grais, depending on their 'orders', reasons for attacking, or other factors.
But, "Oh, I shouldn't do that as DM because it will upset the Player"...er...yeah. Not
really on our radar, tbh.
^_^
Paul L. Ming