I believe it's human nature to either blame someone/something for causing us to make poor/bad decisions in our lives. We even do this when bad things happen in the world. Example: The movie shootings in Colorado. Instead of placing 100% of the blame on the gunman we are now looking at his doctors, the laws on the purchase of guns and ammunition. In fact in the U.S. whenever a tragedy or disaster strikes the hunt begins to find whoever has the "deepest pockets" and sue them. If an employee takes it upon him/herself to be rude or abusive to a customer that person sues the employee's employer along with the individual.
As you noted we have "free will". Therefore there is nothing anyone can do stop a person from planning and executing violent acts especially if that person is willing to die for their cause. The only exception is they are caught in an undercover sting of some kind. People with bad intentions will break the law, blame it on their childhoods, the loss of a job, or a breakup with a significant other/spouse. "It's not my fault! You made me..."