I have been looking at the following two questions recently:
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.
Here are the passages I believe are relevant to answering this question:
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 […]
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 […]
Does the rule on instant death from massive damage apply to monsters?
Best Answer
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:
This isn't an official rule, this is just something "most" DMs do. Probably to make the game run faster or easier.