Ghosts and monsters play a big part in supernatural as the Winchesters profession is to hunt down these creatures. in the context of the show monsters and ghosts are suppossedly evil, but once their reasons are revealed for their actions it becomes unclear as to if they are truly evil.  In looking at the cases involving spirits on the show many of them are not really evil but are merely just angry. Many of the cases involve spiris causing trouble on the date that they died and are acting out as result of the evil that was done to them. The idea of ghosts or spirits of the deceased haunting people or places is the opposite of what christians typically believe, which is that there are spirits but they are not those of people who have died. Monsters in this show are supernatural creatures that have been born that way or were created by someone. The show states that any monster originally came from the alpha monster named Eve, they also refer to any monster being a desendant of the Leviathans. Many of the monsters on the show are represented as an evil character that the Winchesters must defeat. Some of the evil that is represented on the show comes from ordinary humans that are evil instead of from a supernatural creature. The concept of humans being capable of evil is shown in the "Sin City" episode as the town in the episode is over run by the evils of temtation which they  gave into on their own accord. Christian's typically do not like to think of humans as being capable of evil, and that if they commit evil they will likley go to hell if they do not try to ask god for forgivness. Temtation from the Christian standpoint is that it is the work of the devil.