I know that, flavor-wise, force powers with a requirement of "Dark Side" are limited to the "bad guys"... but is there a specific, in-game ruling that defines what constitutes "Dark Side" as a restriction? Is it based off of factions? Characters? The direction the wind is blowing that day? Looking for an actual rule somewhere, not just a "thematic" answer.

(As it stands, explicitly ruled as written, the only card that allows this to be used appears to be the Maul , the Scum crewman. Since there's no other ship that has a "Dark Side" keyword, or other definition of what it means. And even then, only on four potential ships: Kanan Jarrus, both Ezra Bridgers, and Asajj Ventress)

We (still) don't have firm rules yet but the app confirms expectations: it is gated by faction. Imperial FO scum and presumably cis get dark side. Rebel resistance and presumably republic get light side.

I tried to put HATE on Rey - the official app wouldn’t let me do it. So I think that’s how we will find out

I've also noticed the official app allows Asajj to use Hate so the "bad guys" can use Dark Side powers. "Good guys" will need upgrades such as Maul.