The game makes it fairly clear that magic can be used to mind control others across vast distances in space.

Golbez is not an evil individual. Final Fantasy IV: The After Years and the Dissidia Final Fantasy series makes this clear. This question has a parallel in Final Fantasy VIII with Edea and Ultimecia. If you do not hold Edea responsible for her actions, then you should not hold Golbez responsible for his actions.

Regardless, Golbez and Kain understand that there is an inherent weakness within the individual who is being mind controlled, so both of them believe that atonement is necessary.