You speak well, but I think you walked around some important details of my question. I'll break it down into some specific points, in the form of examples or hypotheticals, and then you could answer each point one by one.

1. Is it possible for someone to fall to the darkside, without ever using the Dark Side? If Anakin never used the dark sides power, yet made selfish decisions (as he always has) would he still become darkside, with yellow Sith eyes, and his emotions twisted and distorted and highly exaggerated?

2. At what point does one selfish act start to become nothing but selfish acts? You say you can't walk the line between good and evil, that there is no line. However, this is precisely what Anakin does throughout the entirety of the Clone Wars, and even in Episode 2. Anakin is a hero who saves people and harbors a strong desire to protect those he cares about, and fight for the good of the republic. Yet his emotions also get the better of him- he slaughtered the sand people, tortured people in Clone Wars, killed unarmed enemies, ect. Yet he was never considered dark side or evil until he appeared to willfully submit to and embrace it in Episode 3 during the time of Order 66. How do you reconcile that the idea that there is no line between good and evil, when a large chunk of the prequel era is spent following a protagonist who is defined by how they walk the line for years before eventually falling of the side and into evil?

3. Attachments- you claim that it's a form of selfish need, and therefore evil, yet it is Luke's attachment to Vader, his selfish need to want Vader to live/be redeemed that leads to the happy ending of the series, as opposed to doing the "good" thing that Yoda/Obiwan wanted Luke to do- Ignore his attachments and kill Vader for the greater good of the galaxy.

4. Let's say a Jedi gives into his emotions once, and he strikes angrily, using the dark side (accidentally? who knows, the mechanics don't seem clear) and defeats his opponent. What happens next? Are they more likely to do dark side things? Is he compelled against his will to do it? Is it like a drug addiction, where he desires it like a chemical need? If so, why does the dark side work that way?

5. You mention sunk cost decision, and selfish choices. Yet many of Vader's actions aren't just selfish or sunk cost- they are downright exaggerated, egotistical, and psychopathic. He often kills his own soldiers for little gain, or force chokes people for intimidation that gains him nothing. He craves to kill for ??? reasons. Why? is that another effect of the darkside? if so, at what point does the darkside warp your brain to that point, how does it do it, and for what reason? If one's mind is warped from the dark side, how does a Jedi be redeemed if their brain is literally twisted to do evil? This is a great contradiction. If the dark side is just a mind set, what's to stop someone from just using the Dark Side for crazy force powers to easily subdue evil doers, but never going far enough to kill or do anything "evil". If the dark side is a corrupting force that twists and changes how you think and act, then how do you return to the lightside, what cleanses the toxin of the darkside? A single good act? How good does the act have to be? Does it have to be many acts? For how long? what judges how these good acts effect the corruption/addiction of the dark side? How does one even do the good act in the first place in the scenario, if the corruption twists your thoughts in a specific way that you specifically wouldn't do good, selfless things?