You could do a good movie about Xerxes, but not set in that universe/version of things. It's a matter of timing and what story you tel about him. The way to really make him more sympathetic would be to say tell the story of him uniting that empire to begin with, and the good that came of it and it's conquests... even if we side with the Greeks and support his empire ending given that we have hindsight behind us, and the origins of our own civilization, it is a matter of perspective.
It's short of like how Ghengis Khan and Kubla Khan are presented as being bad guys traditionally for obvious reasons, but you could definatly do some interesting things with them in their own enviroment before the events that lead to their villain status.
I'll also say that the Persians didn't really do anything paticularly bad in 300 overall. Understand it's a war, we were on the side of the greeks. Xerxes wanted Greece, he was a conquerer. He tried to take it relatively peacefully through bribery and failed, he then moved in and tried to conquer it. He's holding his empire together through his personal charisma and perceived invincibility so any kind of serious diplomacy on the spot isn't really viable if he doesn't want to seem weak to those he's commanding.
The Persians slaughtered villages and such, but that's just what a real war is like. The Greeks and especially the Romans were just as brutal in their own offensive wars. Alexander The Great wasn't exactly known for being a nice guy in war, on some levels he's the Greek version of Xerxes.