Savitar = Future Barry
One dies (Iris), one betrays Team Flash (probably either Caitlin or fake Wells), and one suffers a fate worse than death (Barry).
Basically, five months from now Savitar escapes (due to the traitor) and kills Iris. After Iris dies, Barry blames himself and eventually goes completely crazy and out of control. Team Flash can't control him or even keep him locked up and eventually, to protect the world, they have no choice but to trap him in the Speed Force. After being trapped in the speed force for ages, he eventually masters it, sheds his human identity and becomes Savitar. At this point he wants to escape so he somehow goes back in time and uses the Philosopher stone to interact with the real world. Hence the events of the season.
Savitar hates Barry because he hates his past self and knows what he'll eventually do.
Now that I think about it, Savitar pretty much said it. When Barry asked Savitar who he was, what we heard was "I am the future, Flash." but what he actually said was "I am the future Flash.".