I consider possesion and mind control differently, specifically that mind control still allows an individual to think for themselves whereas possession is simply the other person in control. One of the biggest difference between Golbez's mind control and Edea's possession is that Edea has no issues fighting her "children" cause its actually Ultimecia in control, whereas Golbez is unable to bring himself to kill Cecil though he had several opportunities to do so. With mind control, you can't make someone do something they are strongly opposed to doing.

I feel Golbez could be as ruthless and evil and still regretful of his actions without it being possession. Kain is certainly aware of his actions when Golbez controlled him and his actions stem from his own dark desires, which is how he was controlled to begin with. I feel the flashback in the DS version (which TAY has made canon) pretty much sets up a similar scenario for Golbez.

He loves his father and the gifts of his Lunarian heritage, loses his father from human hands, fails to save his father's life cause he lacked power (white magic) and then lost his mother to his brother. I feel all these events could lead Golbez to be secretly resentful of humans, feel superior to them, and wishing to seek power which are all traits he possesses in the actual game. It doesn't mean Golbez is inherently evil or doesn't feel remorse for his actions, but rather just shows its not so different from Kain who simply had his darkest desires manifested. I feel Golbez is the same thanks to the flashback explaining his backstory, as well Golbez being completely in the dark about Zemus' existence and what his actions were really doing.

I agree he partially went with FuSoYa out of guilt (man, they pulled a Krelian) but I also feel that Golbez felt more connected to his Lunarian roots than his human side whereas Cecil is quite the opposite.