Are monsters subject to the massive damage instant-death rules?

Question

I have been looking at the following two questions recently:

• Are Trolls immune to all instant death effects?
• Does the Instant Death rule apply to zombies too?

The first states the following in the question itself:

[...] In addition to such spells, there are more general effects that cause death, mainly massive damage and failing three death saves [...]

That and the answers there seem to work under the assumption that massive damage does apply to monsters.

However the second question's currently most upvoted answer (36 upvotes) states the following:

The rule for dealing damage in excess of your total hit points is intended for player characters, not monsters [...]

These two seem to be in direct opposition and I'm unsure then what the correct ruling is.

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Here are the passages I believe are relevant to answering this question:

Monsters and Death

Most DMs have a monster die the instant it drops to 0 hit points, rather than having it fall unconscious and make death saving throws [...]

Instant Death

Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum [...]

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Does the rule on instant death from massive damage apply to monsters?

Answer 1

Monsters can suffer from instant death

The Combat Rules apply to not just PCs, they apply to everyone. Everyone can move, take an action, attack, etc.

The Dropping to 0 Hit Points section of the rules is explicit. Everyone is knocked out at 0hp. When damage reduces the someone to 0 hit points and there is damage remaining, they die if the remaining damage equals or exceeds their hit point maximum.

The DM can play monsters however they want

At the end of this lengthy section there is a small footnote:

Most DMs have a monster die the instant it drops to 0 hit points, rather than having it fall unconscious and make death saving throws.

This isn't an official rule, this is just something "most" DMs do. Probably to make the game run faster or easier.

Answer 2

Instant Death applies only to PCs unless the DM decides otherwise

The default rule is:

Most DMs have a monster die the instant it drops to 0 hit points, rather than having it fall unconscious and make death saving throws ...

and (MM p.7):

A monster usually dies or is destroyed when it drops to 0 hit points. For more on hit points, see the Player's Handbook.

• If the monster dies at 0hp, the Instant Death rule is irrelevant.
• If the monster doesn't die at 0hp due to a specific ability of the monster (e.g. troll, vampire) then that specific rule applies about when the monster dies - as a specific rule it overcomes any general rule.
• If the monster has an effect that can turn 0hp into 1hp (e.g. zombie) then the monster never reached 0hp (because it dropped to 1hp instead) so the Instant Death rule isn't applicable.

If the DM chooses not to follow the normal rules and treat monsters the same as PCs, then they also need to decide if the Instant Death rule applies to monsters.

Answer 3

Combat rules, including instant death, apply to monsters

The rules for Instant Death are listed as a part of the chapter on Combat. This chapter is introduced with the following (emphasis added):

This section provides the rules you need for your characters and monsters to engage in combat, whether it is a brief skirmish or an extended conflict in a dungeon or on a field of battle. Throughout this section, the rules address you, the player or Dungeon Master. The Dungeon Master controls all the monsters and nonplayer characters involved in combat, and each other player controls an adventurer. “You” can also mean the character or monster that you control.

The chapter introduction clearly states that the rules apply to both monsters and players. This is true for all rules in the chapter including the rule concerning Instant Death.

From the section on Instant Death (emphasis added):

Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.

Since "You" means the monster you control we can substitute it in the sentence to get the rule as it applies to monsters.

Massive damage can kill monsters instantly. When damage reduces a monster to 0 hit points and there is damage remaining, the monster dies if the remaining damage equals or exceeds its hit point maximum.