That free will is good is reasonably straightfoward. One way to frame the argument is the following:
1) Something functioning in accordance with its capabilities is 'good'. (We may or may not like its functioning at all, but that is a separate issue. For example, a good virus is one that performs its function of reproduction well.)
2) Human beings possess the ability to use their intellect to conceive of actions that they foresee will have an influence on the future.
3) Free will would consist of their ability to put into action what their intellect can conceive.
We can see from this that:
1) There is no finite being that has complete free will.
2) That the free will of human beings is further hampered since we do not always do what we wish to do (give up smoking, be faithful, act bravely) even when not compelled by physical laws. This further hampering is ultimately due to our choosing the lesser good (cigarettes, an affair, temporary easing of tension) over a superior good (health, a healthy relationship, self-respect). Choosing a lesser good over a greater one is what Augustine in
libero arbitrio refers to as 'sin.'
The question of the impossibility of evil to create depends on an understanding of what evil truly is. If goodness is the ability to realize potentiality, as we saw before, then evil is the loss of reality, ultimately. Thus evil can not create, since it would involve a contradiction. Another way to look at this is to consider that existence is good, and that therefore evil is that which reduces existence.