What I am

If you knew what I was you'd scream.
I just figured it out.
I have the desire for blood.
But I hunt on all four.
And Human is forbidden.
I change with a full moon.
I can't refuse the moon,for her am victim.
I sing of my hunts,and I hunt ALONE!
I realized what I was,walking by the river on four legs instead of two.
The moon shining bright and full.
A creature larger than a wolf,but a most convenient term.
Blood is sweet.
As is running with the sweet night.
But the sweetest of all is the change, because of the pain.