Does the Instant Death rule apply to zombies too?

Question

When you deal enough damage to reduce a creature to 0 hit points and there is enough remaining damage to overpass the max hit points, they are killed instantly, right?

So I'm using starving zombies for my campaign that are zombies with only 1 hit point. Zombies have the Undead Fortitude trait:

Undead Fortitude. If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

And I have two questions.

1. If you deal 2 damage is it enough to kill these zombies?
2. If so, does the Zombie's Undead Fortitude trait apply then?

Answer 1

The Zombies Still get Undead Fortitude

To answer your first question first: yes, dealing 2 damage is enough to "kill" these zombies. But so is dealing 1 damage. And in either case, they get Undead Fortitude.

The rule for dealing damage in excess of your total hit points is intended for player characters, not monsters. Note that on page 198 of the Player's Handbook (and here in the basic rules), it spells out the distinction between PCs and monsters (bold added):

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. Mighty villains and special nonplayer characters are common exceptions; the DM might have them fall unconscious and follow the same rules as player characters.

Zombies are not typically "mighty villains." Thus, for most DMs, reducing them to 0 hp and then dealing them damage in excess of their hit point total is no more deadly than reducing them to 0 hp. Both events would normally kill them instantly, if not for their Undead Fortitude.

Answer 2

The 'Undead Fortitude' trait closely resembles the Half-orc's 'Relentless Endurance' trait:

Undead Fortitude: If damage reduces the zombie to 0 Hit Points, it must make a Constitution saving throw with a DC of 5+the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

Relentless Endurance: When you are reduced to 0 Hit Points but not killed outright, you can drop to 1 hit point instead. You can’t use this feature again until you finish a Long Rest.

Note how the half-orc mentions being killed outright, whereas the zombie does not. This implies that if the designers wanted Undead Fortitude to not work when killed outright, they would have said so.

Plus, as other answers have said, most of the time NPCs are considered killed outright as soon as they hit 0HP. Undead Fortitude seems designed to prevent this.

Answer 3

It's up to you

As a DM, you can make adjudications in a way you think would be more interesting for the group. Consider following advice from the AL Dungeon Master's Guide:

Always follow this golden rule when you DM for a group: Make decisions and adjudications that enhance the fun of the adventure when possible.

The "Instant Death" rule is intended for using on player characters, not monsters, since only player characters normally gets death saving throws. As a DM, you might use the "Instant Death" rule on monsters as well, if the monster has its own ways to avoid death on 0 hp (apparently, Undead Fortitude does exactly that). It is perfectly fine — many DMs use PHB rules to adjudicate monster's actions (for example, they can say Grapple special attack can be made by a monster).

Zombie probably die from massive damage. The rule says:

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.

The rule explicitly says "you die", not "your hit points becomes 0", thus, does not trigger the Undead Fortitude feature. The Undead Fortitude feature explicitly says it works only when damage drops the undead to 0 hp:

Undead Fortitude. If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

On the other hand, as @JohnMontgomery has mentioned, Undead Fortitude says it drops to 1 hit point "instead". So if it saves then technically the zombie is never reduced to 0 HP and massive damage can't trigger.

Answer 4

It depends on what the zombie's max HP is

I agree with everything said in gandalfmeansme's answer. However, there is an additional angle to consider with these particular monsters. You could consider these to be creatures with 1/1 hp, in which case, the concerns about instant death, etc. do apply. Or you could consider them regular zombies with 1/22 hp, in which case, it would take 23 damage to instantly kill the zombie, which would render the discussion moot as it's not possible for a zombie to get a Constitution save of 23 (using the stat block in the MM).

I personally would go with the latter route, as it simplifies things for you because you can treat all of your zombies the same way, and you just have to decide how much health they have (this also opens up the possibility of half-starved zombies with, say, 10 hp).