This is generally an upbeat series, but there are a few moments that'll make you feel you're watching a horror film.

* Issei's death in the novel was bad enough, but in the anime, viewers get to see him desperately turn to run, get stabbed from behind, and fall to the ground and slowly bleed to death before Rias saves him.
* During a job in Episode 3, Issei comes across the client's room, before stepping in something wet, thick, and red - then the camera pans to show the trail of blood, before Issei [[OhCrap completely and utterly freaks out]]. [[ShadowDiscretionShot We don't see the entire body,]] [[NothingIsScarier but it's so bad that Issei is visibly struggling not to]] ''[[NothingIsScarier puke]]''... and then as if to make the introduction worse, Freed shows us how ''[[BloodKnight happy]]'' [[AxeCrazy he is to have done this.]]
* One of the worst was Juggernaut Drive. Issei goes into an [[SuperpoweredEvilSide insane berserker state]], during which he [[IAmAHumanitarian CHEWS OFF ONE OF HIS OPPONENTS' ARMS!!!]]. He also grows a multitude of limbs [[BodyHorror from all over his body.]] And the real kicker? Ddraig has to warn Rias and the others to run away, because Issei would have killed them as well if they hadn't. The only reason this isn't High Octane is the way he was brought out of it. And this was only from Kiba and the rest's point of view. From Issei's point of view himself? ''[[UpToEleven It's a lot worse]]''.
** The second time he almost pulled off Juggernaut Drive was when [[spoiler: he got himself knocked unconscious by Sairaorg in their match. He then wakes up to a white world with the past Boosted Gear users, surrounded by dark aura. One by one, [[MadnessMantra they tell him to pull off Juggernaut Drive]] to the point ''he joins with them.'']]
* Diodora apparently made a habit of finding devout nuns and getting them branded as heretics. At which point he would rape and enslave them, as no one else would have anything to do with them. [[{{Cuckold}} This is what would have happened to Asia if Issei hadn't saved her first.]] [[spoiler:And all the more satisfying with the resulting CurbStompBattle.]]
* Gasper's face [[spoiler:when he snapped when told that Issei's dead]].
* Kiba's past. [[spoiler: All of his friends dying by being subjected to harsh experiments just so they can hold a holy sword while praying to God not knowing that GodIsDead in this series.]]
* Evil dragons in general. Considered by everyone as the most troublesome existences ever because they just love to destroy ForTheEvulz. Guess they got some lessons from [[Manga/FairyTail Acnologia]].
* Gasper does it again in Volume 16 [[spoiler: when he calmly ''destroys Marius Tepes''.]]
----