Apparently there are a lot of people who think that Savitar (the big bad of season 3) is none other than future Barry.
There are a lot of very good and convincing evidence, reasoning that supports this theory and I believe many of us here heard at least one of them.
However there is a reason why i put an IF in front of my thread opener headline. I am, as always very skeptical about these theories given how most of the fans always gets fooled by the showrunners. We have been fooled in season 1, (well not me, but others) and we were fooled in season 2 (me included).
So IF Barry is Savitar I think that there are several possibilities to how he became or (in my opinion) which version of Barry become Savitar.
Look at some tidbits about the guy from the show:
Earth#3 Jay Garrick said that before Savitar graces a universe with his presence he always sends a forerunner. That was Alchemy/Julien. He (Savitar) wants to battle Flash because he feels that Barry threatens his existence.
Barry did threaten his existence by creating Flashpoint. However in my opinion Flashpoint is also the reason why Savitar exists.
Savitar said that Barry trapped him in the speedforce. Now we know that Barry does not kill his enemies (at least not directly), which is how he manages to walk that fine line between hero and villain.
So I think when Barry "corrected" Flashpoint he "killed" the other him who lived in the parallel universe. The universe parallel to Flashpoint. So I think when the Flashpoint universe got terminated, the Barry who was living in that parallel universe (which is now the main universe) was killed. That's why Barry didn't remember certain events. The death of Cisco's brother for example.
We also know that when a speedster dies he gets transported to the Speedforce, which is a kind of afterlife for speedsters.
So if Barry is Savitar then that's how he got trapped in the speedforce.
Savitar said that Barry took everything from him-->Flashpoint
Savitar said he is the future. Which means that he wants to take Barry's place or he wants to ensure that Barry becomes him/Savitar.
That's why he kills/wants to kill Iris. If Barry loses Iris he completely falls apart. Loses his final strand of sanity that kept him from turning to the dark side.
In the episode 19 promo we can see "flashes" of just how that might began. How does Barry start to walk a darker path, becoming Savitar.
There are other theories that claim that A) Savitar is Barry's time remnant from the Zoom fight
B)Savitar is future Barry
Look at why those theories are incorrect (in my opinion):
A) If Savitar was a time remnant than there were time wraiths who would chased him/or Black Flash. Since Black Flash was chasing Reverse Flash, he is out, but that still leaves the wraiths.
I saw zero wraiths-->Incorrect theory (unless the show runners are deliberately leaving them out to keep us guessing till the end)
B)If Savitar is future Barry then what event would make Barry becoming him and how could Barry wrong himself so bad that he Becomes his own enemy. Furthermore if Savitar was future Barry then why would he kill Iris if he knows that it's the exact thing why he becomes what he is?: A miserable man without friends, family and love. If this theory is correct, then if I were him, then I would certainly not do the one thing that starts the process of me becoming forever miserable. Who in the right mind wants that.
The other fickle thread of evidence that supports this is the fact that Savitar is insane. He went mad twice over during the time he was trapped in the speed force, so this theory apparently might last. I only say it's incorrect because I want it to be incorrect. Because if this theory is true then it's another cheapskate from the writers who supposed to write good stuff and not bad soap operas. If the science of the show is wonky at least get the writing O.K. Am I right? Of course I am!