Orz and the Arilou do not differ that much, and no one of them is "good" or "evil" (as FF&PR3 themselves confess:
<Fwiffo> Oh yeah. Well, I rarely paint someone as absolutely "evil" -- it's all a matter of perspective. What is interesting is why people (aliens) do things that seem evil to you or me, and what prevents us from doing the same.),
though they have different ways, political views. Heed this IRC chat quote:
<Mephisto_> Fwiffo: What exactly did the Arilou *quickbabies* do to piss off the Orz so much?
<Fwiffo> The Arilou always *jumped in front*, which was both a physical taunt as well as a political one. They got to the humans first.
Also, they both want to "change" us according to their views; we are "children" for Arilou and "*campers*" for Orz. We are part of the Arilou "extended family", but Orz wants us to be part of his extended family.
They both have things to hide:
Orz: "Busy is because Orz have reason. It is secret for *campers*."; "Do you want to see our *surprising toys*? No!! Do Not!!"
Arilou: Too many things to quote here.
They both argue in the same way about the others:
Arilou (about Orz): "Perhaps you know of the Orz. Like us, they are dimensional travellers but that is where our similarity ends. Do not trust the Orz, my Human Captain. They are dangerous.";
Orz (about the Arilou): "We are from *outside*. Also the Arilou *quick babies* are from *outside*. It is the same, but not. [...] Orz does not like Arilou. Arilou are too much trouble."