In the novelverse he is an earth ambassador, I agree with most of the novelverse so i guess that he would always choose Cardassia first. Being good or evil is purely cultural, in his cardassian eyes he might think he is a good guy, in our eyes he is morally not so good, but then again, sisko had his moments too
I dunno, I think cases like murder and slavery transcend cultural considerations.
At least, if we don't make exceptions for extreme violations of other people's rights, we are nihilists and should live our lives cynically playing the system and manipulating everything to get whatever we want all the time no matter the effect on anyone else.
But Garak seemed like even though he strongly believed the ends justify the means, he believed Cardassian citizens should enjoy basic freedoms. That's why he protected Quark's girlfriend in that early episode.