Here's what i think - No confusion!

Spoilers ahead

Shaman was a good guy since the beginning. He was possessed when he begged for Buddha's protection and a crow flew right in. Since that moment shaman turned to the evil side.

Monk was a human who sacrificed his whole life for dark magic it seems. Once he died in the truck crash, the evil entity takes over his body completely.

That man in the truck got sick like everybody else. Monk was supposed to raise and fuse the evil entity into that guy because his body is younger and possibly durable. But the cop and his friends killed that sick guy in the process. Now the evil entity had no choice but to take over the dead monk's body. Thus what you see in the end.

That little girl was possessed by a demon and never released.

The young woman, always been a guardian angel.

This is what makes sense to me. Wonderful korean product following the train to busan.