Well, they gave us Nero... who's kinda hunky and ultimately helps us a little in the World of Darkness. I think that's probably the only example though. The problem is the Garleans are dumb. Oh, there's no questioning their military might but their overall strategy is so flawed that it was probably devised on the back of a fast food napkin.
Think of it this way. The Garleans think that by wiping out the Beast Tribes they'll prevent the summonings and therefore destroy the primals. Wrong. The primals exist as manifestations of aether, brought forth by prayer. As we've already seen, it doesn't matter who's doing the praying, whether that's a Moogle, an Ixal or even the five...err, six races. Lady Iceheart herself became Shiva, not merely a manifestation of a primal but a full transformation.
So, by destroying the beast tribes they accomplish nothing towards their goal of wiping out the primals and instead just commit genocide for the sake of it. The worst part is, the beast tribes themselves aren't really 'evil' as a race.
The Ixal only summon Garuda because they were made to believe they would regain their lost flight. It can be assumed these lies were spread by the Ascians. The primal summonings are the main reason they ever came to blows with Gridania.
The Kobolds were at peace with Limsa Lominsa until the Lominsan government breached their agreements and started mining operations in Kobold territory.
The Sahagin were forced from their homes and originally sought asylum from Limsa Lominsa, but were refused. With no where to go they had to make their home where they landed, but LL responded with force.
The Sylphs, while known tricksters, are peaceful forest-loving folk. Even the tempered few don't go out of their way to harm others unless you invade their territory. They summoned Ramuh because of Garlean invaders.
The Amal'jaa are largely tempered individuals, forced to do Ifrit's bidding. Since their focus is on summoning their primal deity, Ifrit, it's fair to say their offspring are brought up to worship Ifrit for the most part. The untempered few, however, have shown to be very honorable and consider tempered Amal'jaa their enemy.
So, of all of the beast races, the Amal'jaa are the only ones you could consider 'evil', but it's not really their will but rather the will of Ifrit being invoked through them. The Garleans, on the other hand, have shown no mercy to any race and seek to conquer and enslave that which they do not destroy.