This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Doug Walker
The few times that Doug himself has gotten legitimately angry. Being used to the Nostalgia Critic character, seeing Doug actually go real angry can be scary in his own right. Cases in point: his real review of Twilight: Breaking Dawn Part 1, and his retrospective on the worst movies he reviewed as the Nostalgia Critic. Particularly Garbage Pail Kids.
He stated in the 2013 drive that he's scared himself by getting so angry at (non-movie) things, but we haven't seen those moments and never will. If the above is toned down but still scary, then it's even worse knowing he can get even madder.
The two Forced Viewing podcasts he was in. The first one has him telling a friend (who saw his friend get shot) that it'd be interesting to see someone hang himself, and the second has him getting really upset that nobody else likes a b-movie that he enjoyed.
It's bad in other videos where he can't hide in the dark, but his neck in the Iron Man 3 Sibling Rivalry. It's showing muscles and sinew that really shouldn't be seen, and just distracts from what he's saying.
The indiegogo video is oddly unsettling, with dark shadows, weird camera angles and Doug in full Ask That Guy mode.
An in-universe example that doubles as Fridge Horror; Doug's "Disneycember" review of Alice in Wonderland notes that when you think about it, it's very likely that people really did get decapitated offscreen in the film, and that if Alice didn't watch her mouth around the Queen of Hearts, the same would happen to her.
While his relating to Zuko (and Azula in "Boiling Rock", and Aang whenever he had guilt) in the Avatar vlogs constantly was more of a Tear Jerker, "understanding" Mai's urge to kill out of boredom, identifying with Jet's need to kill innocents, admitting he'd be a bully with fire powers and "would need to think about it" if he had destructive power that could kill him, at the very least raised some eyebrows.
He surely didn't mean anything bad by it, but it's still not hard to understand why some were freaked about by his mini-musing in "The Library" about war being good for sorting out the weak.
With the "body violation" (sub)text of bloodbending, a lot of people were skeeved by how he really thought it wasn't a bad thing to do. And to a lesser extent, because of how much weight he's lost, him saying starvation isn't the worst that could happen.
The suicide wishing 'joke' in “When Extremes Meet” wouldn't nearly be as uncomfortable if it wasn't about two minutes long and played completely straight.
The Star Trek: Spoiler Corner gets weirdly Adult Feary, with Rob comparing the bad part of the movie to abusing his own child, and Doug's countering that it's more like someone he loves telling him he's not good enough.
The ending of the "Gutgrinder" vlog. Jason is joking that he watches Doug while he sleeps but Doug actually looks terrified, even telling Jori when she giggles offscreen that it's not funny.
In “Blood Under The Skin”, Rob holds Doug down and Jason gets a cleaver to chop his head off. For a few moments Doug looks genuinely freaked. Ages later in a Daredevil vlog he explained that he has a weird fear of knives.
The Mood Whiplash at the end of "His Hero", going from sweet "let's go adventuring" to Doug screaming "don't touch me there!" offscreen.
In "Mystery Train", Doug gets a little too excited about the prospect of killing the Laws cat. As usual, lampshaded by Jason who says he has nothing to do with it so don't get PETA on him.
In "The Limit", Doug's manically desperate laughing as he tells Jason that he abuses his Appa plushie. Then Jason protects the Finn doll and Doug sadly says he's sick in every meaning of the word now. It's awkward timing too because the video released beforehand (the 2013 holiday message) had him resting his head on the plushie and thanking the fan who gave it to him.
The end of “Another Way”, where Doug laughs like the clown nurses and his manically scary face gets right into the camera before the episode cuts.
The ending of “Red Throne” was made for it, with Doug going between Stepford Smiler “happy family” and screaming at Jori while she cries.
Also in “Betty” where Jori said he scared her and he acts abusive and shouting again. Although it's made easier by Jason putting a stop to it and giving him an excuse to leave.
While most of the "Kyle Hebert is an A-Hole" video is hilarious, the part with Kyle chasing Doug down the hallway while narrating was strangely scary.
While Doug makes a joke about it being a jolly death, Rob bringing him down by the neck with Christmas lights and choking him is still pretty shocking.
In the fourth Daredevil vlog ending, the sudden-ness (plus for most of the vlog he was subdued and Rob pretending to beat down Doug is always expected) of Doug grabbing Rob's shirt and screaming Fisk's line in his face. Even Rob has a WTF expression.
In the Alice in Wonderland real review, Doug's loud rant about people who think the movie is faithful to the book, slamming his hands down on something offscreen and even making Rob back off and go “...um”. Of course Doug being himself, he apologizes (and again outside the review) and feels bad for exploding.
In the Gravity Falls finale vlog, Rob screams "Waddles!" in Doug's direction probably louder than intended, as even Doug shrinks back.