A good man out of the good treasure of his heart brings forth good things, and an evil man out of the evil treasure brings forth evil things. But I say to you that for every idle word men may speak, they will give account of it in the day of judgment. For by your words you will be justified, and by your words you will be condemned. Christ teaches here that choices come from the heart. A good nature will result in good choices, and an evil nature will result in evil choices.
Good trees produce good fruit, and evil trees produce evil fruit.
Our words and actions, therefore, are not determined by an autonomous will, but rather by the nature of the tree. Consequently, Jesus says, someone with an evil nature is incapable of speaking good things. But this inability, this bondage, is caused by the nature of his own heart. It is not external compulsion. Evil men are therefore free to do what they want, but they are not free to do what they should.