no matter how they dress it up as superior species.... they are providing a service for humans... so by all definitions during the whole service they are servants
and its not a favour because its not something the angel can do just by the by
he has to search for them fight them and kill them thats the same service as your local assassin guild
so in essence those angels are debasing themselves from their high horse of superior species without realising it... making them effectivly about as smart as a dog