Fly-By-Night, And Day!

Fly-By-Night, And Day!
Twas a horrible creature from hell
Black as the night, and mean in flight
Hell-bent, with evil intent
Whilst spewing on its prey
Purifying its victims flesh
Before the decay
Like a vulture, from the depths of Hades
Where Goblins danced their mad charades
Whilst Cathedralithic Gargoyles
Watched on in sinister glee
At tortured souls
That no longer see
This creature from Hell
Had Hellish eyes
With a thousand mirrors turning
Reflect a hungry souls yearning
And a Devils desire
 For a sacrificial burning
And come what may
Whenever the day
To turn around in revulsion
And dismay
For this creature
Although its life is brief
(And for that, we should never grieve)
Has a population, that far outnumbers our own
And one day, perish the thought
(And so we ought)
Grow to such extremes
(Or so it would seem)
That one would scream
For i find
Not wishing to sound unkind
That even one of these ghastly creatures present
(I'm sure their intention is malevolent)
I feel such disgust
At their revolting lust
To prey on discarded remains
It may come as no surprise
When one realised
That these creatures are responsible
For death and diseases
And anything that displeases
But WHY
Is it simply called
A fly?