Cas is a ghost hunter/killer. It’s kind of a family tradition- his father did the same thing until he was murdered by one of the ghosts. Cas wants to continue his father’s legacy, so he and his mother- a white witch - move from town to town in search of dangerous ghosts.
As soon as Cas meets Anna, he realizes that she is not a typical ghost. He saw her kill people and knows she is very dangerous. Yet he also senses she feels regret about her actions and doesn’t want to murder people. She doesn’t seem to be in control- something is compelling her. Anna even warns Cas to leave her house before he gets hurt.