This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Digimon Frontier
When Takuya turns into his Beast Form for the first time, he turns into a mindless berserker, even lashing out at his teammates and comes within an inch of killing Tomoki. The horror in his voice is clear even in the English dub where while his voice is calmer, it's clear the realization of what happened is a true kick to the gut and it's no wonder he's terrified of his Beast Form for a while after that.
Kouji Taking the Bullet for Takuya when his plan for Duskmon ends in Punch! Punch! Punch! Uh Oh... was already bad enough, given that he not only returned to human form but also was passed out from the pain but in a following episode, Kouji is clearly holding his ribs while trying to rescue the others, meaning that they are at the very least badly bruised and probably worse since his breathing is slightly off as well.
The climax of the Mercurymon arc has Mercurymon nearly stuffing Takuya into a coffin and killing him.
As hilarious as his voice acting is, there's just something chilling about how playfully murderous IceDevimon is. Even worse, his dialogue implies that those... things in the basement of Cherubimon's castle were the result of him feeding on the data of Digimon that he captured.
Duskmon is actually a pretty brutal Deconstruction of the Adventure series. Take a random human child with strong emotions, drag him into your world against his will, and make him fight for you.
Lucemon's voice actor in the English dub voiced Izzy in season 1 and 2. For anyone that grew up with those two seasons, they'll recognize that voice anywhere when they hear it in any other character in the later shows. But with Lucemon, they go even further, giving him a more evil tone that's very disturbing. You're basically listening to an evil Izzy at this point and he's ready to destroy the world.