I am glad that I could widen your scientific horizons, haha.

Well, If I were a supervillain, I would probably use such technology - but with some modifications. I would use light-sensitive proteins that respond to infrared light. Infrared light has higher penetrative capabilities so maybe that would be enough to activate the proteins without the necessity to actually drill hole to the skull. Next thing - the choice of neurons - I would probably use something that would paralyze the vicitim, like pain centre or balance centre or something like that. There is one major problem - how to deliver those genes of interests? Maybe use some virus or some modified mosquitoes or something.

But! I think that there far easier methods to achieve mind control than to use optogenetics.. Thus I thing that even though it has a potential to be weaponized or misused as every technology it will not happen. And humans would also probably develop some immune reaction to the foreign protein expression or so. And you would need some great external source of light - which is also a bit hard to achieve.

What I think might happen - we might be able to identify the workings of the brain to better understand them and thus cure variety of neurodegenerative diseases or depressions. Maybe we could be able to delete unpleasant memories or make you relieve some memories - if you think about it everything in our minds has to have physical basis - every memory, everything (of course only if there is no soul or something like that - which sadly probably is not). We could target and destroy cancer cells with light, we could activate some genes with the flick of a switch. Actually why being limited only to light - there are sonogenetics that use sound to control neurons, or other approaches that use radio waves.

Phew - I got carried away a bit :D But to summarize - I do not think that this technology is going to be actually misused. We have far more effective methods to control people - money, fear, upbringing, religion.