What is free will and can there really be a pure form of it?

There isn't such a thing as a "pure form" of free will. Our will isn't completely free, except we have enough freedom to choose to do good or to choose to do evil, that moral choice is completely free. There may well be a genetic predisposition towards certain sins but that doesn't mean the person isn't free to resist that particular temptation, we are not slaves to our flesh acting on every impulse without thought, we are free thinking sentient intelligent beings not mindless impulsive animals (actually not all animals are mindless and impulsive, even some of the lower animals have demonstrates some signs of intelligence).