When a person has what we call bechirah, a person has to chose what his actions are going to be based on his on perceived view of the world around him. His choices are based on the limited knowledge and understanding that a human brain has. There is no way of him knowing or calculating how his actions would affect the world. And, dare I say it’s ludicrous for us to think that a human being’s actions can affect the Master Plan Hashem has worked out.
Hashem in his infinite wisdom knows exactly the choices that are/will be set before a person and knows how they will respond. That doesn’t take away our bechirah. It’s just that Hashem is the one who created us, our brains and our imagination. He’ll know exactly what we’ll do when. He was the one who wired us in the first place. It’s juvenile to think that we can somehow outsmart our own creator and sort of put his plan in disarray.
Think about a professional painter working on a large painting. He knows the exact amount of which colors to mix to achieve a desired tone/hue for that spot. You’re not going to have red all of the sudden take on a blue color because that’s not the nature of red.