Skull Reaper, the very last boss that they fight, just before the reveal that Heatcliff 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 entire 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 even some of the players start murdering each other. Basically, 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.
When people come out of their virtual imprisonment, both in SAO & ALO, they're disoriented, their muscles are atrophied, etc. They're helpless. Think about what would've happened if Kirito hadn't stopped Sugou's knife attack outside the hospital where Asuna just woke up. After he'd already declared he was going to rape her while comatose.
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. And of course, 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.
Not to mention that 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 even 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. A Complete Monster indeed.
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 just about 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 basically 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 more scary 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!
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.
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.