Cross-posted from Tumblr, as usual because I keep forgetting this is an account I have. There's some interesting commentary on the Tumblr version of the post; this is just my original half-formed ramble that could probably do with a more thorough analysis.
Vader’s suit as a method of control is an interesting thing balanced against the typical standard that his immoral actions are his own and he must be held responsible for them. That suit has an off switch, for heaven’s sake, it’s Slave Transmitter 2.0 for Anakin in that respect. He’s had his bodily integrity yanked out from under him, and under those parameters he can hardly be considered a free agent.
But it’s indirect control. I’d imagine he kills a great many people over the course of his time as the Emperor’s dragon that he’s never explicitly ordered to kill. And the general torturing that goes on is probably even more excessive. He’s not having his every move dictated by someone with a remote control.
And that suit is so solidly baked into him, removing it could well be catastrophic. The role he’s been forced into is so deeply entrenched in his thinking that he probably couldn’t just snap out of it if he tried.
Plus he did do something independently to make him a Sith Lord in the first place, the suit came later. Implicit slavery comes very naturally to Anakin. The suit may well have been overkill on Sidious’ part, given Anakin’s past, but Sidious isn’t one to take unnecessary chances.
Vader may honestly be a slave of his own head as much as the suit, and it makes delineating between what he has responsibility for and what he doesn’t a little tricky. In the end it all seems to come down to the fact that he never learnt how to disobey.