Why would a Redguard choose to support the Empire? The Empire abandoned Hammerfell so Redguards are naturally hateful of the Empire. Furthermore, one of the books is written by a Redguard who says they admire Skyrim, not the Empire so it would make sense that a Redguard would support the Stormcloaks. In fact, Ulfric says that he considers you family during the Stormcloak questline so Stormcloaks definitely have your back. The Empire would be willing to throw you under the bus if it meant peace like they did to Ulfric during the Markarth incident.

As for the second part, I just have to facepalm. You are either joking or are seriously biased toward the Empire as the Empire has more to do with Nazis that the Stormcloaks.

As always, it depends on your characters own opinions. Does he believe the Empire deserted Hammerfell and wants to fight against them? Does he believe a united Empire is best for everybody? Lore can only go so far. Your character's own thoughts are far more important than any lore.

That said, the stereotypical Redguard would probably fight for the Stormcloaks. They forced the Empire to denounce them, and drove back the Thalmor without the Empire's help. Relations are ... tense between Hammerfell and the Empire.

And then there's http://www.uesp.net/wiki/Skyrim:Lu%27ah%27s_Journal, who has... er, strong opinions, but not necessarily coherent ones.

It depends, if you want Hammerfell to become a new Empire, crushing the Stormcloaks before they can become a threat in the future is a motive, or allying with them to defeat the Empire and Thalmor is another motive.