So I'm reading Darth Plagueis, and I enjoy all the backstory and Palpatine and how he apparently devised every scheme in the entire original trilogy from the Trade Federation to Camino, but one thing I don't get: why did he turn into an alien-hating fascist? There's no real indicator of why he and/or the Imperials hate aliens so much.
In the old EU, it was to explain why the Empire in the OT was composed exclusively of white dudes. The EU Empire was also racist and sexist (the one prominent female Imperial, Daala, got there only because she slept with Tarkin, another icky plot point I'm glad is gone from canon), but this is no longer the case. There have already been plenty of female non-white Imperials introduced in the new canon.
In current canon, Palpatine himself sees
everyone and everything else as beneath him, and has no problem using anyone for his own purposes, no matter the race. The Empire itself is officially not species-ist, Mas Amedda is still the Imperial Vizier at the time Rebels takes place. However, he allows systematic alien discrimination because it serves his purpose - he loves pitting different factions against each other, his reasoning being that as long as they're struggling against one another, they're not plotting against him.