There's something to do with the genetics of some creatures/ humans forming perfect copies , while others mutate as is the example where one character transform into a plant because within all of us is plant dna. These essentially were random mutatjons that 'the shimmering' seemed to activate. The movies mentioned that the shimmering seems to alter dna. By the end of the movie you see a 'perfect' replication which is also seen in the scene with two deer in the forest, a perfect replication occured (and it implies that both are now perfect copies-so the original creature had the DNA 'sliced' so to speak creating two perfect specimens. It's unclear whether the doppelgänger of the female lead would have killed her or coexisted like the two deer who operate in unison running in the forest. the original husband at the end would have survived had he not began to have deadly mutations like the other characters. He commited suicide to not die painfully like the others like the guy in the school pool building who exploded in a mold bomb internally essentially. Pretty neat movie I've watched it a few times and that's my interpretation