AMBIGUITY

One aspect of LOST left open to personal interpretation is whether Jacob was good and MIB was bad.

MIB seemed to represent the "dark," while Jacob the "light."  But the relationship between the two is more gray.

A key component of ancient Egyptian burial rights is the story of Set and his brother Osiris.

Set is portrayed as the usurper who killed and mutilated his own brother. Osiris' wife Isis reassembled Osiris' corpse and resurrected him long enough to conceive his son and heir Horus. Horus sought revenge upon Set, and the myths describe their conflicts. The death of Osiris and the battle between Horus and Set is a popular theme in Egyptian mythology.

In the temple, there is a painting of the smoke monster sitting across from Osiris, the god of the underworld. They look at each other on the same level, which may represent equality in the spirit world. 

In one interpretation, Jacob could be the Osiris character while MIB the boastful brother Set. In the island world, Osiris would have taken revenge on his brother as Jacob did with his brother. As a result, Set is transformed not into a sun god, but a smoke monster, to continue their underworld battle.

Another interpretation could be that Jacob is Horus, the nephew of Set, who set out to revenge his father's death. The crocodile statue of Tawaret represents fertility so the island can be considered a place of rebirth of Osiris. So it is possible that the smoke monster is a transformation of the underworld god (who can shape shift) while MIB is is also a disembodied spirit. Jacob, as Horus, the sun god, must be weary of the night in the underworld where he is most vulnerable. So the dynamic between Jacob and MIB is one of caution.

In either case, they are not purely evil or purely good.