This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Salvation War
Kills entire human cities with just his mind. Needless to say, this scares the hell out of a lot of humans, and he is marked as a priority target. The guided missile cruiser USS Normandy "sacrificed" itself (having to spend months in drydock) after its... particular way of wounding him, and in the final battle over Los Angeles, target differentiation and even "avoiding other aircraft" becomes a secondary priority to his death.
Those he kills don't reincarnate in Heaven or Hell and may not reincarnate at all, even if there is another afterlife "above" Heaven and Hell. This terrifies Angels and Demons alike.
However, Michael states that it is more likely that, rather than completely destroying the souls of those he kills, that Uriel simply sends them to another place besides Heaven or Hell
The Leviathan in Stas Bush's side story Don't wake me while I'm quiet.
The writer doesn't shrink from describing exactly what modern weapons do to the people/beings they maim and kill. That was an editorial policy decision; if people want to see their favorite weapons used, they ought to be aware of what those weapons do to their victims. Case in point: the Russians breaking out sarin to take down harpies and nagas on the Phlegethon and dealing with the backfire on those troops whose chemical seals had been compromised beforehand.
Looking back on some of the earlier chapters in Armageddon about human warfare (specifically them being hounded and pursued when Abigor's army collapses and flees), the language used to describe the human pursuers, or rather their armored vehicles and attack helicopters, actually veers in the direction of Survival Horror and even Humans Are Cthulhu.
This obviously extends beyond the weapons: bear in mind that every single human in this story goes to Hell upon death and the tortures that they are subjected to, such as being crucified or thrown into rivers of lava, barring only the highest percentile of those who'd shed every part of their will except a desire to kiss Yahweh's ass.
The greatest horror: Aborted fetuses appear in hell and are sold as delicacies.
Dust storms, especially as experienced by two survivors of the Dust Bowl.