Garnet says multiple times how things changed once a "strnage man" came to teh castle. That was Kuja. He corrupted Brahne, filling the void of despair she had after her husbnad died with a manic greed to control the world. Also, she killed thousands. Easily. Just because we don't see the full population, doesn't mean they weren't there. It was the graphical limtations, obviously. Unless you want to suggest that there are only about 10 people in Rocket Town in FFVII....
Kuja used her but it all comes down to speculation and your opinion. Just because someone is controlled, does that mean that they are not fully responsible for their deeds? What is to say some part of her didn't truly want to do all Kuja aided her in doing? And Brahen went after to kill him because..that's what power-mad tyrants do. They keep gaining power and they forget that that power was only attained through help. They forget that they'd be nothing without that person's aid. So they think they, with the power this person helped them obtain, they can destroy that person and thus solidify their unrivaled power. Kuja predicted it from the start because it's pretty obvious and he's pretty smart.
Kuja used her but it all comes down to speculation and your opinion. Just because someone is controlled, does that mean that they are not fully responsible for their deeds? What is to say some part of her didn't truly want to do all Kuja aided her in doing? And Brahen went after to kill him because..that's what power-mad tyrants do. They keep gaining power and they forget that that power was only attained through help. They forget that they'd be nothing without that person's aid. So they think they, with the power this person helped them obtain, they can destroy that person and thus solidify their unrivaled power. Kuja predicted it from the start because it's pretty obvious and he's pretty smart.
He was smart in ACF and he's smart here too 
