Hallow and the manga feature fairly graphic depictions of a dog's death, though the first scene of this dog involves burying it, so it is not unexpected.
Allen is used as a servant by his master but it’s largely comical. His backstory is a little darker but it doesn’t show serious physical violence. (There is a gag with a hammer being used to knock him out, though). Lenalee’s backstory shows a rough childhood of her being restrained and tested on - so that may be difficult for some people.
Allen’s eye is cursed by a scratch from a demon but it isn’t graphic and is just shown as a scar he has. His eye does change when activated and occasionally plays up, but it isn’t mutilated.
Posession becomes a major plot point starting after the Japan arc. There are multiple other themes of bodily autonomy being overridden by external sources.
The main monsters have weapon appendages that could be seen as guns.
Two characters fight with magical guns and regularly point them at each-other's heads while speaking.