He feels the lure when Qui-Gon dies.
He's hanging there, and the monster is gloating, and Obi-Wan's angry. One moment of anger, and the monster is dead. Qui-Gon is avenged. It was easy. So very easy. Just one moment of anger.
From then on, whenever he fights, he feels the call of power, darkness. He feels it everywhere, and it sickens him. It terrifies him. It sings to him with empty words, and it's so tempting to accept it.
Some days all he can think of is how easy it was to kill that monster. If he'd just found that power sooner, no-one would have died, everything would be fine. When he thinks that way, it's harder to resist the darkness than any other time. Because it would still be useful, it would still shorten battles and save lives.
It would be so easy to give in. It would be effortless.
He spends his life resisting that call.
When his padawan accuses him of hiding behind the Code so he doesn't have to feel, he almost laughs. The boy is right, of course. He hides, he always hides, because he knows who his true self is, he knows that his heart has tasted the darkness and will never shine again.
He isn't scared of emotion anymore. He knows it too well for that.
By the time Anakin is Knighted, every hint of his inner conflict is hidden behind a thousand shields, so deep that even he can occasionally forget it exists. Nevertheless, he fights the Council appointment every inch of the way, because he isn't worthy, isn't close to worthy, and if they understood that they'd never press him again.
If they only understood the cost of fighting Sith.
He considers revealing it to them, letting them see why he's the worst choice in the Order, but he can't face the disappointment that would follow, he can't face destroying the hero everyone sees him as, so he finally accepts the appointment and hates himself a little bit more.
If you can call it hatred. Hate needs fire, and the dark inside smothered any sparks long ago.
When his world collapses around him and the darkness comes back, stronger than ever, he again feels the call, as compelling as it was the first time, but this time, it's almost easy to ignore. He knows this territory better than he knows anything else. He understands the darkness, and it holds no more temptation.
During the years he spends in the desert, he's almost grateful for that first taste, all those years ago.