Bombs are bad enough.
Francois: Do you know what kind of a bomb it was?
Clouseau: The exploding kind.
- from The Pink Panther Strikes Again
It's even worse when it's the exploding kind of monster.
Deadly Death Throes
While I refer to them as "exploding monsters" not all of them explode. Some melt in puddles of acid and take your weapon with them, some turn to stone, some do some kind of death blossom when finished off. In any case, it makes dealing a death blow to the monster potentially fatal, even if non-fatal blows don't otherwise harm you.
These kind of monsters are pretty straightforward - attacking them is fine, but killing them causes them to death a horrible backlash against their killer, or at least against those that surround them. Usually this takes the form of blowing up like the monster was made of thermite and nitroglycerine instead of flesh and blood.
Fans of Hardcore Diablo II will think about beloved characters who hit the wrong Stygian Doll at the wrong time and then had to stare at the screen showing their dead character.
Why do they explode?
There are broadly two types of excuses for a monster that blows up - physiology, and magic.
Physiology is the old "it's a big ball of toxic/noxious/explosive/flammable gas" monster, or "it's made of unstable negative energy," or the old "its blood is combustible upon exposure to air."
Magic explains the rest. The demon explodes because, well, f--- you guys for killing it. The witch has a dying curse. The draconians dissolve into acid for no reason you can tell. More technologically you have "it has a self-destruct device inside of it" or "we've got explosives!" - which for all practical purposes will work exactly like magic.
Most of the variations of this I've seen, used, or thought up vary the death throes, and include:
- Exploding additional effects (napalm, fragmentation, cold)
- Curse - either as bad luck ("It's bad luck to kill a wizard." - Malak), a disease, or a straight-up supernatural unhappy but temporary doom.
- Pool of Acid/Slime/Puke/whatever
- Death Blossom/Retributive Strike (basically, attacks everyone in an area for a few turns before dying)
- Poison gas
You get a split between fratricidal and non-fratricidal death throes, too. Some exploding monsters set off their friend's death throes, causing a huge series of explosions. This can be handy if you get one and kill off the rest, but equally it can be disastrous if you're caught in the overlapping explosions.
Non-fratricidal monsters have a more limited damage range. On one hand, you won't get the overlapping explosions bit. On the other hand, you can't use clever tactics and positioning to dispose of a bunch of them at once. The GM can get more mileage out of fratricidal explosions than out of, say, fratricidal curses or turning into pools of dangerous acid or poison gas clouds.
Using Exploding Monsters
The decision between fratricidal and non-fratricidal, as well as the nature of the death throes/explosion, is critical.
For a GM, the difference affects how dangerous the monsters will be. If each and every Giant Gas Bladder Monster explodes and kills all that surround it, it'll be a quick encounter either way if the monsters all hang out in each other's blast radius. If they attack with more spacing, they might not be a big of a threat. PCs will benefit from tactics and ranged detonation tactics. On the other hand, monster might deliberately deploy exploding monsters to soften up the enemy - send in the Poppers and then follow them with your normal troopers. Hard on the exploding monsters, but their lifespan is short, anyway.
On the other hand, with non-fratricidal monsters (say, mummies with death curses), the GM doesn't need to worry about how they deploy beyond any normal tactics. They simply won't bother each other with their deaths. But the PCs might need to worry more - 10 mummies means 10 curses. Is it better to space that out, or have one guy deal 10 finishing blows and hope it's easier to de-curse him 10 times (if he lives . . . ) than spread out the effects? Or if it's damage, do you sacrifice one to save many, or try to spread out the damage?
In my personal experience, exploding monsters are a lot of fun - they add a lot of tension to encounters. Equally, players will moan about them but monsters that explode feel more fragile (and often are more fragile), since so much of the threat is tied up in their death throes. In GURPS, they're much less hardy (explode at -1xHP, instead of a chance of death and automatic death at -5xHP). You get that "fighting a glass cannon" deals it/can't take it feeling. It makes for a shorter but high-tension encounter.