After all if אדם הראשון had no יצר הרע how could he do a חטא? We can ask an even more basic question. Did he even have בחירה?
R' Dessler explains based on many sources as follows. Before the חטא he was certainly a בעל בחירה however on a different level. The יצר הרע was outside him. There was no "I need, I want". Instead it was "you need, you want". The Rambam explains that before the חטא everything was אמת or שקר, everything was black and white. To do a חטא was like to walk into a fire, it was crystal clear that it was the wrong thing to do. There was no way to rationalize or justify. After the חטא however, things are now טוב ורע, a much more subjective reality where an עבירה can turn into טוב and we can rationalize our actions.
If so, how could אדם be חוטא? The answer is that he willingly walked into the fire. He felt that his status made things too easy. If everything is crystal clear where is the נסיון? If רע is so clearly false then there is no challenge in staying away from it. He felt that he could create a bigger קידוש השם by eating from the עץ הדעת and having a greater נסיון to follow the commandments from Hashem. He wanted to be able to understand רע and stay away from it. Unfortunately, he miscalculated. He did not understand that eating from the עץ הדעת would bring such confusion on the world. Someone at his level could not fathom the world after the עץ הדעת, he could not fathom the חשך of טוב ורע.