Wills does not necessarily mean "Decides" but rather "Allows"
Here's a comment from AndTheEgyptianSmiled
This topic is kind of heavy for many people, to the point that some scholars advise to avoid it if it continues to confuse them. I'll break it in 2 parts, followed by an example and a brief summary, and inshaAllah you'll be satisfied.
Let's define destiny ==> In arabic it is called "Qadr" and Qadr is what Allah decrees to occur. Allah gave man the freedom of choice. Some things are outside our control (our birthdays, who our parents are, the sun rising and setting, etc.). Other things are within our control (our hobbies, the words we say, the clothes we wear, the fasting, the praying, etc.)
Now Allah's knowledge in NO way interferes with our ability to choose. Allah is beyond time, and in fact, He created what we call time. His knowledge is perfect and as a result, He knows everything that will occur....including the choices we will make. Just because Allah gave us the ability to make choices does not mean He approves (or rejects) our choices. In the end, life is simply a test - and the only thing that matters is our intentions...Allah created us, He created our circumstances and our surroundings and gave us the freedom to obey Him or reject Him.
If you respond to my post, you'll have the freedom to write anything you want. Allah has given you the gift of a mind and the ability to express yourself freely - what you write to me will be completely up to you.
And even though Allah already knows what you will write, His knowledge of this will not interfere with your words - and that's why you'll be responsible for your actions. Or, to be more accurate, you'll be responsible for your intentions.
Once something has occured, then know for a fact that this was Allah's will for it to occur. This does not mean Allah approved of it, it just means He decreed it.
In brief, here are 3 rules:
a) We have free will to make choices. We are responsible for those choices.
b) Allah's decree does not interfere with our free will & choices.
c) The actions we will take are already known to Allah, for He has knowledge over all things.
tl;dr Think of Destiny as a sort of knowledge, nothing more. This knowledge that God has does not interfere with man's ability to choose (between right and wrong, good & bad, etc.).