The duality of nature exists in heaven also. Lines in the sand are just that. Allegiances change and thus the physical. Falling from heaven and rising from hell happen much more commonly that thought. This particular angel is in process of tearing away it's heavenly form in favor of something much more suited for it's new role in hell.
Death is not one, but many. Over countless millennia, they have been chosen for their selfless sacrifice. Each soul taken has a price, a drop pf poison that slowly turns the angels into shadow, until they are no more.
Angels, or angelus communia, have no shape or form, until they allow themselves to be seen. The figure is nothing more than what its' seer imagines.
Guardian angels are common angles that have chosen to protect certain humans. Sharing the same attributes as their brethren, they are more earthly bound and thus have certain sentimental attachment to earthly delights.
Not all that fall, fall to the dark realm of hell. In fact, most are kept out of sight in the heavenly ether, hoping that they will see the error in their judgment and once again return to the light.
One of the more ancient of the angelus species, its power is strictly for the most heinous of crimes. Rarely seen, its power is nearly limitless, as is its rage.
Angelus custos, or guardian angels, are mainly earthbound entities, rarely returning to the heavenly light. The extended time in the presence of the ones they are sworn to protect also heightens their emotions, and are prone to melancholy and despair.
When mankind first discovered fire, it was praised as magic and god-like. In way it was, as the first of the angels created were element based. As they were sworn to protect humans, their images came in the forms of what was most comforting in the dark times.