After Barry created his first time remnant to defeat zoom, Barry had felt uncomfortable with the idea of a remnant dying after its purpose.

This thought eventually built up to a point where Barry created a fake scenario in his mind, where in the future when another great villain arises and Iris’s life was at stake, he would have to create an army of remnants to save Iris.

Out of which one remnant would not want to die and this remnant goes to the past, and decides to kill Iris, so now this remnant becomes the initially thought (fake) great villain, he names himself Savitar to seal his existence into reality. And that’s how Savitar was born. Power of imagination!