Flynn became a part of the monster because he wanted to see the "truth." By seeing the truth, he saw every killing/murder in Echo where the murderer got away, literally everything. Because of this, his "soul" is able to see all of the truth/every death in all possible timelines. The monster represents the "truth", and Flynn became a part of the monster, and he is stuck in a loop to see the truth repeat itself over and over. He is living all timelines all at once.
Basically, multiverse.