The Source is the good of the island, The Monster is the bad by RandallFlagg

A simple theory here. A lot of people seem to be a bit underwhelmed by the creation of Smokey so I'm going to put in my two cents.

So the "source" of the island is the heart, and contains life, death and rebirth. An all encompassing force too extreme for us to comprehend it's power.

Perhaps when The Man in Black fell into the source, this provided a vessel for all the bad things that the source represents to escape as a physical form. Like Jacob said in Ab Aeterno, "hell, evil, malevolence". The human DNA of the MiB mixed with the source to create a manifestation that can think, have memories and most importantly become separate. That's what the Monster is, a physical manifestation of all that is evil.

Maybe if Jacob fell in, the monster would be a little nicer.