QUOTE THE DEVIL

As Jacob says in Ab Aeterno,

"There’s many other names for it too: malevolence, evil, darkness. And here it is, swirling around in the bottle, unable to get out because if it did, it would spread. The cork is this island and it’s the only thing keeping the darkness where it belongs."

Good and evil coexist on the planet. Human beings can be extremely kind (sacrificing) and extremely cruel (destructive). Temptation and sin is common place. So it is not like the "forces" of evil are not already present in our character's world.

Malevolence is the wish to do evil upon others.
Evil is the attempt to harm others (with immorality) as embodied or associated with devil.
Darkness is wickedness, distress, gloom with the lack of spiritual or intellectual enlightenment.

Why the island is the great gate between our world and projected dark chaos is the number one mystery to be explained by the ending.