Skull Reaper, the very last boss that they fight, just before the reveal that Heathcliff was actually Kayaba, pictured to the right. No wonder everyone was running away in fear.
It's actually much worse than it looks. It killed several players with a One-Hit Kill attack.
The premise of the Aincrad arc. 10,000 people think they signed up for a game, and instead they were really signing up for a war, and they don't even get to go through basic training first. As many of them die in the first month as do in the next two years. Everything Is Trying to Kill You, and some of the players start killing each other. The entire Aincrad arc is scary.
In A Murder Case in the Area, Grimlock arranged the murder of his guildmaster and real life wife Griselda because she actually rose to the challenge that SAO provided and she was no longer as submissive and dependent on him as she used to be IRL.
The opening to Episode 15. Everything's nice and calm and peaceful. And then the world breaks.
Poor Asuna. Her example actually crosses with Fridge Horror if you analyze what she had to go through. First off, it is implied she may have had no experience with gaming ever before, since, in her misfortune she got into SAO out of curiosity by borrowing her brother's NerveGear (who could not play on the first day), only to find out she is stuck for over two years in a virtual world where there's death around every corner. Just look how mentally broken she is when introduced early in the story. And then, after SAO is completed, what next? Instead of finally achieving her freedom, she is imprisoned inside another game by a friend of her family, who turns out to be an Ax-Crazy pervert threatening her to rape and brainwash her into submission. Kirito is the only thing that keeps her sane.
Now think about Asuna's brother. He lends his little sister a game while he was away, only to find out that she was caught in a trap meant for him. His situation is obviously not unique. How many parents and older siblings gave Sword Art Online to children as a gift? How many of those children died because of that gift? Survivor guilt ain't limited to the SAO players.
When Asuna's former escort reveals to be an Ax-Crazy killer who almost suceeded in murdering Kirito. All with a poison that paralyzes him. Thank goodness Asuna had Kirito on her friend list.
If Sugou hadn't been stopped by Kirito, he would have raped Asuna, who was in a coma, in her hospital bed.
The threat of rape is bad enough. But then there's also that he was almost certainly planning to use his mind control technology to make her love him (or some sick alternative) by manipulating her emotions and altering her memories, which may also have included completely erasing her memories of Kirito! And there's no telling what he might have done to Kirito himself. In fact, this whole scene is also kind of a Tear Jerker, as well.
Sugou's injuries from Kirito curb-stomping him in ALO were pretty gnarly. Before said beatdown, Sugou warned Kirito, while torturing him, that when the game's pain inhibitor dropped below level 3, it would affect his body in the real world. Then Kirito gets administrative control over Sugou and set his pain inhibitor to zero. When Sugou appears in the real world trying to kill Kirito, he's going partially blind in one eye and has dove off the deep end. Granted, the son of a bitch had it coming, but still...pretty gnarly.
Death Gun. To the average person, the idea that they could die in a game is already frightening. To SAO players, it's living through the infamous "death game" all over again, especially since two of the people behind Death Gun were members of Laughing Coffin.
What's even scarier is the secret behind how Death Gun kills. Someone breaks into your house and gives you a lethal injection. If your consciousness is inside a game, what's protecting your body?
And the threat doesn't have to be malicious, either. What happens if your house catches fire? You won't even hear the smoke detector!
It is pointed out in the Fairy Dance arc that Amuspheres are designed so that if you get called by family police hospital or fire departments you are automatically pulled out of the game and in the Mother's Rosario arc we learn that loud noises such as someone shouting at you cause a warning to appear in game to tell you to log out.
The deaths of the Black Cats, specifically Ducker, who is swarmed by Dark Dwarves who immediately butcher him with their pickaxes, with Ducker screaming horribly with each hit until he finally dies.
Even worse when their leader, who was off buying their new guild house, comes back, finds them all dead, and leaps into the abyss to commit suicide.
Anti-crystal zones: Traps where deadly monsters attack players who are unable to retreat because their teleportation crystals don't work. It gets worse: they're not anti-teleportation zones, they're anti-crystal zones, which means that healing crystals don't work either. SAO was already dangerous enough, but these traps (including some boss chambers, which are unavoidable) disable a player's two most important lifelines: the ability to flee by teleportation and the most efficient means of healing.
Sinon's Blood Knight tendencies are bad enough that Kirito actually suspects her of being Death Gun.
The look on Kyouji's face as he's proposing to Shino. It's at this point it becomes clear that he's lost it. Shino did tell him to wait for her and was probably sincere, but she wasn't expecting him to be this when the time came. After she pushes him away, he decides that since she won't have him, he'll just rape and kill her, then kill himself to join her in the next life, but not before confessing the entire Death Gun conspiracy to her. It's more terrifying when you look at his face before Kirito manages to save her in a nick of time.
The most unsettling thing of the above is that Shino was almost raped by someone she had known and trusted for years. Even worse, unlike the situation with Asuna and Sugou, she got along really well with Kyouji and enjoyed being around him. It reflects the grim reality that many, many rapes in real life are done not by complete strangers, but by somebody the victim is well-acquainted with.
The fact that of the Siblings in Crime in the Death Gun conspiracy, the one who came up with the idea was the one who wasn't an SAO survivor.
Kirito's severe depression in Red Nosed Reindeer. Even before he turns explicitly suicidal and resolves to banzai charge the bosses, he's displaying multiple warning signs of contemplating suicide: brazenly reckless behavior, insomnia, and cutting himself off from what few friends he has. Even scarier is the fact that this stuff is real: severe, untreated PTSD can easily degenerate into severe depression and suicide.
Infinity Moment/Hollow Fragment
The increasing frequency of traps in the last few floors count as Jump Scares even if the player has little chance of actually dying.
The situation with Sugou/Alberich is even worse than in canon. Not only is he able to abuse GM privileges to sexually harass and kidnap players like in ALO, he can now take advantage of the fact that he's in a Deadly Game to actually kill people via a slow and incurable poison. Worse yet, Kayaba is too busy dealing with glitches and such to stop him, meaning the raid team would have been screwed without Strea's Heroic Sacrifice.
Asuna worries that if all the blame is shifted on Kayaba, people like Sugou could get away with their crimes and retaliate against them. She's Properly Paranoid considering the post-Aincrad events in canon, especially in the case of the Laughing Coffin.
In the sidequest to recruit Hollow Sachi, you have to enter a room that looks suspiciously like the one where the real Sachi died. Then you have to fight off two mobs that can easily kill Hollow Sachi if you didn't level her first, effectively recreating that nightmare scenario.
How about sleep-PK in SAO? Towns and other settlements are supposed to be safe zones where nothing can lower your HP, other than dueling, which requires your explicit consent. Thus, while within one you can rest easy, knowing you're never in danger. And this is critically important when what you initially thought was only a game has become something that could really cost you your life. And then, one day someone discovers a way around it, which involves exploiting the duel system: just by sneaking up on you while you're asleep and moving your finger to touch the popup window, they can fool the system into believing you've accepted a duel, and proceed to murder you while you're most vulnerable. And the worst part? Unlike in the real world, you might not wake up even while under attack.note It doesn't help that players are not made to feel any pain, which, while undesirable for having fun in a mere game, could be a life saver in this scenario. So now you have to live with the knowledge that even in a supposedly "safe" zone, if you've pissed off the wrong person, or have an item worth killing for, or you're just plain unlucky, you just might go to sleep and that would be the last thing you ever knew.
In Season 2, Episode 20 Asuna is suddenly disconnected from the game, The screen turns to a white tunnel and fades out, she comes back to reality to find her mother standing over her with the power cable to Asuna's Amusphere still in hand and angrily reminds her that she warned that she would "pull the plug" next time she is late for dinner. It doesn't seem to occur to her that "pulling the plug" might have some unfortunate implications to a girl who had been traumatized by over two years of being comatose or that barging into her bedroom while she was unconscious might also trigger some trauma since Sugou had threatened to rape her while in a similar state.
He did get it back from Seijiro. Since Seijiro's unit handles all the fallout from SAO it's mostly safe to assume they have some techies who disabled that backup battery and brain fryer. Also, unlike SAO, other VR games probably have no coding that prompts the brain fryer to function upon player death, as they are built for the Amusphere, which has no such feature.
PoH's final fate is filled with this. While Kirito's brutal smackdown of PoH is awesome, the way he finishes the fight is also scary: Kirito turns PoH into a tree and tells him to pray that his allies will bail him out, otherwise he's stuck in there, rotting for the rest of his life until someone else cuts him down with an axe. Kirito shows no remorse for the bastard and even admits to Sinon flat-out that he didn't regret doing it.
Made even worse when PoH, former leader of the Laughing Coffin, becomes terrified as the branches sprouting from his body are wrapping him. As Kirito puts it, PoH will spend ten or more years of isolation and insanity, and then die after another 30 years of brain activity.