The title of the series alone speaks volumes: The Metal Gear is absurdly terrifying once you realize what it can do. The thing launches nukes that could reduce a city to ash in seconds, but unlike a missile silo, it moves. And if you try to shut it down, it fights back. It can launch its nukes from anywhere on Earth... and they're completely undetectable by radar. If these things really existed, we would have to live with the knowledge that nukes could fall from the sky at any moment without warning. Most games would leave this as simple Fridge Horror, but Kojima is absolutely blatant about Metal Gear's implications because, well... Some Anvils Need to Be Dropped.
Metal Gear leans hard into horror and fear in a military setting more so than any other similar series. All of the stories are set within the shadow world of ultra black ops, layered over the top of the "normal" world. This drives the sense of alienation and altered reality many characters experience. The average person is a pawn of a vast global conspiracy, pathetically ignorant, and the main group opposing them are war loving fanatics, obsessed with perpetuating a world of carnage without end or purpose. Why? Because because the shared trauma of fighting other people's wars have broken them, making them unable to live normal lives. The various heroes are emotionally and psychologically devastated, alternatively either loving, or loathing, the fact that the battlefields of the world are the only places they can call home. The bosses of the series are the world's deadliest killers, usually outclassing the heroes and driving them to the brink. Throw in literally monstrous doomsday weapons, mental deterioration, and regular villain victories, and it's amazing any of the heroes emerge from the horror sanity intact.
Alternatively, you try to take out a guard from behind with a gun. You forget that your gun doesn't have a silencer. !
You're in Evasion phase and trying to find cover. The backup team makes a thorough, thorough check of the room, and they manage to find your hiding spot. Once again, !
Many came to dread the "Game Over" screens in Metal Gear Solid, where a random character will scream for Snake after his death ("Snake, what happened? Snake?! SNAAAAAAAKE!!").
Of course, this is a subjective trope for a reason.
Then again, the cries in the original Metal Gear Solid are probably the worst in this regard. Hearing Mei Ling or Naomi scream SNAAAAKE! with utter anguish is heartbreaking and terrifying in how real the voiceover feels.
MGS4's game over is even more disturbing: in addition to the "Snake? SNAAAAKE?!!" line, the most vital scenes from the game's story up to that point flash rapidly on screen as Snake dies, as if to remind the player what was at stake.
Plus, the music that plays during the game over is so fast paced, it makes it more nerve-racking.
Gray Fox: Imagine dying, then waking up in a painful exoskeleton, and experimented on repeatedly, with your memories coming and going and the only sensation you can ever have is pain, the poor, poor guy.
Imagine a man who can turn invisible at will, deflect bullets with a spin blade spin who can suddenly and terribly go Ax-Crazy while you are unable to land a hit. You never felt more pity for any Mooks than the ones he slaughters just before you fight him.
No matter how badly Raiden is looked down on by the fandom, he really did not deserve what happened to him between MGS2 and MGS4. He was eventually kidnapped and put through a similar stage of torture and experimentation that Gray Fox has been through, only worse - he loses everything below his upper jaw and it gets replaced with cybernetic parts. Fortunately it seems that he hasn't suffered the same psychological trauma that Gray Fox did.
And for those who feel sorry for the poor guy, at the end of the game you find out that he has had surgery to look human again, but there are still marks from the cybernetics visible.
Vamp is a Depraved Bisexual with the ability to jump from any place, anywhere with great skill with that knife of his, an extreme blood-addiction to the point where he even ripped the throat out of a SEAL, and he can pin you into place by using your own shadow.
Don't forget the fact that he was caught in a church bombing, pierced by a crucifix, forced to lie in the rubble next to his dead parents, and only survived by drinking their blood.Fun times!
While this is not a canon game, it nevertheless has one of the creepiest villains in the series, Marionette Owl. He was born with a condition that let him see in the dark and became a Serial Killer after finding his childhood friend cut to bits and strewn over a field by another serial killer. He was recruited into the Black Chamber unit after he was cornered by the FBI, but was given a pardon by the team's leader, Black Arts Viper in exchange for working for them. He fights against Snake in complete darkness while using two large marionettes. It's revealed that they aren't normal marionettes either. They were created using the severed limbs and body parts of his prior victims, and during the fight with Snake, comments that after he kills him, he's going to use his clavicle and femur bones to repair one of his marionettes. And then he goes to Hell for being a monster... and his last words show that he knows it. Rated E for Everyone, folks!