The Dark Side is a lure that corrupts the user mentally and physically, the deeper they go, the harder it becomes to break free. It is temptation and corruption, something to make people stray off the path through means beyond morals as non-Force users do not experience it. The Dark Side from its aura alone if strong enough, can break Jedi in its presence and turn them over, like how Revan used Malechor 5 to turn Jedi to his cause. While surely, the Dark Side starts with a choice to delve into it, what continues onwards is often uncharacteristic of the person before it. While not exactly manipulated, its that certain parts of them are magnified, often negative parts like zealous hatred or a thirst for power. It is not the same thing as a thirst for power that normal people have, it is compulsion given to them by the Force, compulsions uncharacteristic of them.
Now, this isn't always the case. At some point, people simply give into the darkness. People that truly harness the darkness within them without remorse, with full intent, and with complete control. Sidious, perchance? Vader was under stress of trying to do something out of love and passion, he didn't care about the dark side then, he was just trying to save Padme. Vader wasn't making the choice to go the dark side, he was making the choice to try and save Padme, and through that, was led astray.
And beyond that point, it can be argued that they are still in control. They do the things they do willfully, just without inhibitions. Its like someone on a drug trip against their will murdering a building of people. In the end, their actions are understandable, but they still need to face the repercussions of what they've done. They can be forgiven, but they still have to work for redemption. Is it their fault for being drugged? No. They don't deserve punishment, they deserve redemption, rehabilitation - under great inspection and scrutiny, behind bars still, but at least not with full-blame upon them.
Though, this isn't entirely accurate, as Vader still made a choice, and the person in the analogy didn't and was forced to. But we say, that person was forced to take the drug without exactly knowing the full consequences of it or else the person with in the camera would slit the throats of their family. Still, the analogy still feels kind of loose looking at it.