None whatsoever.

Bete Noire's pet Akumu and her pink blob monsters kill people by violently stabbing out their souls and then consuming them.

Though it's not worse than a TV-14 rated show, the series nonetheless doesn't hesitate with blood and violence.

In Episode 7 Season 2 , A character is badly beaten up by his own wife, but he presumably survives.

In Season 1 Episode 4, a character had his arm ripped off.

In Episode 3 season 1, Chara cut off his own arm.

Frisk, the main protagonist, get killed multiple time by Sans in the first episode.

Season 1 is passive violence, but season 2 is more violent.

There are some uses of hell in the series.

None.

Much of the characters Undertale often had a dark side to them. Being a fan work of Undertale, this series wastes no opportunity diving into these dark pasts throughout the series, some of which are quite traumatic.

The series has gained notoriety among the Undertale fandom for its high body count, with significant number of the main characters being killed off, much to the shock and sorrow of its fans.

Bete Noire very often makes frightening faces.