God's will overcome ours,exactly like the story of virgin Mary when god planned to bring Jesus to the world through her,no one can stop it(Jack in your example).
In normal life,god has nothing to do with our free will,its our responsibility for the choices and decisions which we take during our life,so god has nothing to do to help the drug addicts because that was their choice,but still god can help if he wanted to,we never underestimate god's power to do whatever he want.
So, in other words, we can do whatever we please, UNLESS what we please is going to cause, somewhere down the line, part of God's predetermined plan to fail.
For example, let's say the little girl I see walking down the street with her mum will have a child who will have a child who will have a child who is a vital part of God's plan. I can't do anything to stop that little girl I see today from having kids, such as killing her, right?
But given that the smallest action today can have far reaching consequences in even the short term, doesn't that mean that
everything must be a part of God's plan and can't be altered?
Here's another example...
I work as a train guard. My train is running late, and I'm trying to get it back on time. Normally, if I see someone running towards the train, I'd wait a few seconds for them, but because I'm late, I'm not waiting for them today. As a result, this man running for the train misses the train. He decides to catch a bus instead, but as he is crossing the road to get to the bus stop, he is hit by a car and killed. Or perhaps he was destined to meet his future wife on that train, but because he missed it, he'll never meet her and fall in love and have kids, which is a shame if one of those kids would have grown up to be the person who brings about world peace.
Now, if that man was part of God's plan, then obviously he can't be killed in a hit and run, right? So therefore it must be a part of God's plan that I wait for the man. It certainly isn't a choice on my part, because I can't do anything else but wait for the man, right? If it was a choice, that would mean I have several options to choose from, but in order for God's plan to be fulfilled, I MUST wait. No choice.
Another example?
I'm at a crowded shopping mall. A man nearby decides to start shooting people, and many are killed. I was in a position to stop this man, as I had just brought a new frying pan and I could have hit him in the head and stopped him from shooting anyone, but I was busy sending a text message to my girlfriend and I didn't see him. Now, if it was God's plan for all those people to die, then it MUST have also been God's plan for me to be distracted with the text message.
See how complicated it gets? SO how can you say that some things are part of God's plan and others aren't when EVERYTHING is connected together?