
For the former, the Garlean Empire is more widely known to most players than the Allagan Empire, particularly for newer players or players of 1.0. Garlemald uses Magitek, whereas the Allagans used Allagan technology, but I think the biggest indicator of all would be that the question uses present tense, more or less ruling out the Allagan Empire as an option.
As for the former... good and evil is always subjective, at least to some degree. I think many people might think a nation that conquers other nations by force (seemingly with world domination in mind) then essentially brainwashes their people to assimilate them into their armies for future conquests and uses violence and force as their first option in any situation might be considered evil. And that's not even getting to the bit about crashing the moon into the earth, nor cohorting with Ascians and Voidsent, or purposefully summoning the Primals to absorb their power.
Either way, seeing as the game is set in Eorzea and we all play as Eorzeans, the narrative we're fed tells us that Garleans are evil, biased as it may be.
As for the former... good and evil is always subjective, at least to some degree. I think many people might think a nation that conquers other nations by force (seemingly with world domination in mind) then essentially brainwashes their people to assimilate them into their armies for future conquests and uses violence and force as their first option in any situation might be considered evil. And that's not even getting to the bit about crashing the moon into the earth, nor cohorting with Ascians and Voidsent, or purposefully summoning the Primals to absorb their power.
Either way, seeing as the game is set in Eorzea and we all play as Eorzeans, the narrative we're fed tells us that Garleans are evil, biased as it may be.