- Some players jumped a little upon first seeing the character select screen for Metal Slug 2/X & 3.
- A lot of the artwork is pretty disturbing. Think Monty Python animations, only in a war time setting and (mostly) not played for laughs.
- Metal Slug 2 (or X) is where the supernatural elements start coming into play. They're not too bad until you reach the subway passage route and end up having to make your way through a mutant infested area who will explode in an attempt to take you out.
- Mission 2 of both Metal Slug 2/X and Metal Slug 3 in general. Not only are the levels filled with undead beings (mummies in 2/X, zombies in 3), said undead beings can turn your character into one of them (granted, your character still remains controllable as a mummy/zombie, if much, much slower). Mission 2 of Metal Slug 3 adds more nightmare fuel with the level music, which sounds noticeably dark and creepy fitting for a zombie-infested plane crash site, caused by an alien meteorite.
- In a combination of this, Squick, and Nausea Fuel, several of the zombies attack with infectious vomit but only two variants (female zombies and one of the civilian male zombie types) of them puke it out. The others expel it via violently tearing open their ribcage (fat zombies), squeezing their head until it pops out of their brain (the other civilian male zombie type), and most gruesomely pulling out their guts like a hose and squeezing it out (doctor/scientist zombies).
- The Bloody Murder special attack zombified players get: The zombified hero vomits up a horrifying amount of blood.
- The zombie Rebel soldiers are especially terrifying with a much more menacing look about them, and unlike the slow moving ones, will actually bum rush you in a suicide attack that can turn you into a zombie if it hits. Thankfully, they're only found near the end of 3's second level.
- They also appear in the final stage. To be more specific, they're zombie clones of your captured comrade: I.E, zombie clones of your friend. And by virtue of being clones of a playable character, they get the blood vomit attack, too. Making things worse is that said segment is you on the run to escape the now self-destructing mothership and having to destroy barricades as the zombies keep coming. On single-player, this is both horrifying and utterly difficult. Metal Slug 4 also features both mummies and zombies in Mission 4 (granted, you only encounter one type depending on the path you took).
- A lot of the deaths are animated in disturbing detail; lighting people on fire in particular.
- Special mention goes to the animation for when you get hit by a zombie's attack if you're already zombified in Metal Slug 3's second level. It has the player character's head fall off. And if you're in the air while it happens, your character explodes like the rest of the zombies' death animations.
- Every time a stage is accompanied by "Metamorphosis", you know some disturbing things are about to come down.
- Metal Slug 3D's final stage, Lieu. It is a Womb Level with wet flesh everywhere. The cutscene after you beat the robot boss in the first part also counts.
- The Avatar of Evil, the Final Boss of Metal Slug 5. It is an entity that comes from nowhere, has a hellish screech, and it leaves just as quickly as it appears. It even looks nightmarish, with its red eyes, Celestial Body, and it has a giant human skull on its chest, as well as the giant scythe it holds. And what even is this thing? Is it the Grim Reaper? Is it some sort of demon god? Or something else entirely? The fact that there is absolutely no info on this thing doesn't help at all. Perhaps the only comforting thing is that it is driven off by normal human weaponry note .
- On the subject of final bosses, there is Rootmars, the leader of the Mars People and the final boss of the third game. For starters, he is not exactly the prettiest alien you'll ever meet, with special mention going to the fact that his brain is visible and sticking out of his head. His entrance is rather unsettling as well, given how he flies in from nowhere and cackles madly before doing battle with you. That said, the fight afterward will likely give you dreams of a much better kind.
- 6's version of "Final Attack", which is essentially a Dark Reprise of the aforementioned theme. It starts out like every other version of the theme, only for it to change into something more creepy and otherworldly-ish.
- The events of Metal Slug 1 to 6 happened in the span of three years. In other words, the six world-war scale conflicts against the three NGO Superpowers (Rebels, Amadeus, Ptolemaics) and two different Alien Invasions attempting to Take Over the World happened an average of six months apart from one another. Bearing in mind these conflicts also involved: a zombie outbreak; an alien impersonating a major world leader; giant mutant animals created by mad science; an alien invasion so severe the heroes were forced into an unholy alliance with their arch-enemies and the other aliens (who had only just failed their own invasion attempt); and a cult wielding both advanced technology and functional magic with a literal Avatar of Evil in its ranks. Whilst it's mostly played for laughs, in-universe it must be a horrible time for anyone living there.
Nightmare Fuel / Metal Slug