From what I know of Zermus, he was just a traditional FF villain. Kill everyone, conquer everything..Complete Monster kinda thing.

Garland was the first example of moral ambiguity in the FF series as far as I'm aware. Sure he wanted to kill everyone and take over the planet but that's what he was made to do. He is doing what he thinks is right and necessary to save his people.

Even when Terra is destroyed he does all he can just to save the universe. Though his entire mission, all his hundreds of years of existence, were for nothing, he helps save everyone else.

Garland, like most of the FFIX's villain cast apart from Zorn & Thorn, had a lot of depth.
I'll give the writer of the article credit for understanding that much.