Simply put, the Garleans are Imperialists. Their driving force might be the same as ours, but their MO is literally "Because they can't do it right, we will save them from themselves."

Yeah, by CONQUEST.

It's also been mentioned that they seek to emulate the Allagan Empire, and we ALL know how much of a pleasant lot they really were. . .

Sorry, but Imperial rule and being a Warrior of Light just don't mix.