This is a golden oldie, I did this picture for an assignment in my high school English class in 9th grade.  I forget exactly what the assignment was, but it was something relating to Paradise Lost which we were reading at the time.  The angel has been sent to strike down the demon, but he takes no pleasure in it.  This angel feels sorry for the demon he's striking down, since demons were once angels after all, but he doesn't have a choice.

I'm actually in the process of redoing this picture, check it out here!