Tanya herself. Imagine a little girl whose personality is not too dissimilar to that of Brock Sampson from The Venture Bros.. Not only that but she's constantly making rather disturbing faces.
Manga version of Victoria's depiction of war in Rhine frontline, in which she describe it as an experience where she lost her humanity and become something worse than trash. They're all squirming, covered in mud, blood and ash. They couldn't move at will, screaming before knowing what's going on before becoming dead silent.
As a newbie, Tanya threw Visha and her new flight straight to trench where she tripped and bumped into a headless torso that's still warm. Tanya didn't even flinch before ordering her to move forward if she wants to live, and when Visha fought a Republican soldier Tanya came from behind and cut her opponent in two, showering her in blood and guts. By the end of close battle, Visha is on her knees and vomiting as Tanya congratulates her for surviving.
In the perspective of Allied Kingdom's volunteers, Tanya and her battalion invoke this. Only a small number of them survive and are haunted by the experience. Even in light novel and anime, it's not depicted in full as the surviving officer could only remember a vision of burning hell and scream hysterically. One of the lines says one of their mages lost his mind when he was still on air and pursued without mercy by Imperial Mages.
The sheer Moral Myopia of the series. Much like real life conflicts and especially the socio-political atmosphere of World War I, excess amounts of patriotism and honor, and few being far-sighted enough to realize the consequences causes all sorts of hell. The Empire is invaded first out of fear of its rising military strength by its neighbors, but the moment they fight back and overcome the odds, everyone else surrounding them suddenly invades, creating a total war scenario off of branding the conflict against the "evil" Empire as a Black-and-White Morality conflict. It's not that the Empire is innocent - it's that they're forced to take over opposing nations out of self-defense, using that same military strength they're feared for and increasingly ruthless tactics to get its conquered nations to stop fighting forever, with all the bloodshed that entails.
It's only amplified by how Imperial soldiers are treated when captured. How does Tanya reach a snapping point that causes her to wipe a city off the map, civilians included? By seeing the local militia (who are essentially unlawful civilian combatants) torturing, abusing and coldheartedly gunning down their Imperial prisoners of war. This gets overlooked by the enemy nations altogether, and the city's destruction is instead focused on to demonize the Empire further as a means of fanning the flames of patriotism and war... even though if the Empire even appeared to do something similar, everyone would be all up in arms.
Mary Sue seems like a cute and adorable young girl. Then her father dies, and she goes off the deep end to kill Tanya. The consequences of this haunt both her allies and the enemies for a while to come, and, a good deal later in the Light Novels, she's butchered and hacked apart by her own comrades because she was considered too insane to be allowed to live combined with being a potential further threat due to Tanya's shift over to the US.
The manner in which she receives her powers is quite creepy: her dying father, her mother and she are visited by three different incarnations/avatars of Being X, and told God has decided to grant three miracles. Her father wishes for a better future for her, giving her an unbreakable will and a shining destiny. Her mother wishes for her to be protected, giving her divine protection and even regeneration. And she wishes to have strong magic like her father, to become a soldier. The concentrated three wishes turn her into an utter monster in the battlefield.
The manga follows a green Empire recruit as he reaches the meatgrinder that is the Rhine front and joining up with Tanya's unit. He witnesses as the few remaining Empire captives are killed by the Republic army and partisans, and terrified, he hesitates to fire back. Tanya deliberately floats over to his side and coldly points out that as horrible as it may seem to him to kill Republic forces, each casualty prevents further Empire deaths, and hammers the lesson into him so he won't hesitate the next time. As Tanya keeps the defenders pinned, the artillery strikes begin, starting the slaughter of Arene and leveling the city.
Lavrentiy Beria, a pedophile sexual predator (hence his charming nickname "Loliya", from "loli" resp. "lolita complex" (i.e. pedophilia targeting young girls) and "yandere" (an extremely obsessive kinda love, often violently so)), who has targeted Tanya for both her body and brutality in war, wants to capture her and force her to become his bride.
This character was inspired by the real-life Lavrenti Beria, head of the NKVD, torturer and infamous serial rapist and killer.
, torturer and infamous serial rapist and killer.