They may be easy enough to kill, but these demons are very high level and very skilled.
A slayer demon feeds off of guilt. It’s an emotion they cannot possess, and thus makes it the most delicious emotion for them to taste. For them to feed off of someone’s guilt they most have a soul, thus often creep up to the mortal world as the underworld is filled with demons and other soulless beings.
The feeding of the guilt can either cause complete mental break down as the guilt is often intensified before consumption or can drain the being of all sense of guilt voiding them of remorse and conscience.