"Vader was going to be extremely powerful, but he ended up losing his arms and legs and became partly a robot. So a lot of his ability to use the Force, a lot of his powers, are curbed at this point because, as a living form, there’s not that much of him left." [George Lucas; from THIS post]
This quote is rather enlightening on the meaning behind Old Ben's "He is more machine now than man" line in ROTJ to explain that Vader is "twisted and evil", something I always found a bit out of the blue. While the OT is rife with themes of nature over technology, and technology being used to control and harm the people, the ultimate evil of the trilogy (Palpatine) is entirely organic. Now, I read Ben's quote more as a commentary that being so removed from the Force has only worsened Vader's evil.
I do understand what Lucas is going for with his construction of "more machine than man" Vader being weakened and twisted by his state. Vader is on life-support in the OT and general understanding is that the Force lightning killed him by disabling said life support. He is 'the walking wounded' or perhaps more accurately 'the walking dead' -- his life ended on Mustafar and it's just misery and evil that keeps him going. We understand that he's barely human anymore.
The problem comes from the fact that there are real people out there who are missing limbs and surviving thanks to medical intervention, and to say there's something inherently "twisted and evil" about this state is ableist. It also raises questions about how much of your flesh you can lose before your experience a power loss, which is a very dry way of looking at things. (Come on. One arm = 10% power loss? What do you think?) And, obviously, it can contradict "luminous being we are, not this crude matter" and the whole existence of Force ghosts, which prove that the body is hardly necessary to be powerful and present in the Force.
What I would say is that, as with so much of the Force, this is entirely a matter of the spirit, and perception of self. When Anakin lost an arm, it didn't slow him down. So what if that's it? The prosthetic is a part of him that he accepts. He still considers himself whole, so he is still able to access the Force fully. Then Mustafar, and the spiritual death of Anakin Skywalker. He's lost everything. He's a monster. He's left in a permanent state of chronic, debilitating pain. He has no choice about being put in the suit, and the only thing that he was holding onto -- Padmé -- is revealed to be lost to him. From this point on he is in a state of spiritual unbalance, and he never has a chance to feel whole again. His body is no longer a vessel, but a prison, an unwilling, perpetual walk forward when he's lost what makes him Anakin Skywalker.
He defeats Palpatine because Luke leads by example, showing him what peace and acceptance in face of great suffering looks like. He's empowered by his son, and able to reject his own apathy, pain, and imprisonment to act. He's not interested in being the walking dead any longer, and so he chooses to finally die. This is a narrative that doesn't rely on him being in the suit, but rather the suit becomes a metaphor for.
With that, we move from, "he's twisted and evil because he's more machine than man", and into "he's twisted and evil because he's rejected his own humanity/personhood/spiritual peace."
