The Free Will Defense against the Problem of Evil is relatively unique compared to the more common responses. Most responses fall into the general category of "God does not have omnibenevolence", and some are that "God does not have Omnipotence", and a few reject the concept of agency -- that God would act to implement His will. This defense holds that the basic Problem of Evil is correctly stated, but that there is a previously unrealized property of Free Will that makes it a Good that counteracts all evil.
This argument, if one spells out its premises, has the following form:
- The Problem of Evil is a valid argument, God would create the best of all possible worlds to achieve His objectives, and there is great evil in this world
- However, we are mistaken about the nature of Omnibenevolence, and Free Will
- Free Will is so important, that its existence vastly overbalances all the evil in this world
- IF we have Free Will THEN we will do great evil
- And this evil we do of our own free will is the sole Evil in the world
- Therefore this Good of Free Will makes this the best of all possible worlds
There are a variety of ways to critique this set of assumptions.
Your critique is entirely valid, although it is only one of several problems with this "free will" argument. God, as you note, IS free. Yet does not do evil, at least per the omni-benevolence theology. You are offering a falsifying test case, demonstrating that Plantinga's rationale is missing a few key elements.
What the main flaw you are poking at here is, is that what one does is not just based on freedom, but also upon inclinations. One can be entirely free to do massive amounts of harm, but also have no inclination to DO harm. Such a person would, entirely freely, do no harm to anyone. So -- what inclinations do we humans have, that we do harm often? Clearly we are such that we have a significant propensity to commit harm, yet an omni-Creator COULD have created us without such a harm-leaning propensity.
This refutes decisively one key point in this argument: "IF we have Free Will THEN we will do great evil".
So -- if we could have had just as much freedom as we do now, AND be more loving, caring, and helpful than we tend to be, THEN God as creator would be responsible for all the evil we do because of the inclinations that we have, and God is once again the responsible party for the evil we do.
There are several other objections to this argument, which address other flaws in it. One of those is that this argument, which presumes that the "good" of free will trumps all the significant evil that it presumes free will requires to therefore happen in the world, does not address that our human free will is very limited. We have limited knowledge, power, imagination, and ability to control our own inclinations, AND we are highly constrained by the choices of others. Humans could be MORE free if we had more knowledge, more power over the world around us, stronger imaginations and judgment about how to achieve our goals, stronger wills to overcome and/or mold our inclinations, and more autonomy relative to the constraints other humans put on us. IF free will were SO morally valuable as this argument presumes, that even our limited free will is worth more moral weight than all the evils of this world, then any step to increase our free will should similarly be such a great good as to outvalue any other considerations, and any creator God should therefore have maximized our Good of Free Will. Any creator who left us with such a limited portion of free will, would have created an immensely morally deficient world, compared to one in which we could have had so much greater freedom, leaving this world once again far from the morally best of all possible worlds.
Additionally for a third objection, most of the evil in the world is not created by humans, it is intrinsic in the Darwinian equation. Life multiplies until it is living on the ragged edge of survival due to scarcity. AND life can only survive by destroying other life. Our universe is structured to cause scarcity, competition, and massive quantities of violent death. This is evil built into the universe at its foundations, and no degree of human freedom or lack of freedom has any effect on its intrinsic moral imperfection.
So -- your critique is valid, plus there are at least two more that are also highly effective in refuting Plantinga's argument.
What you will likely find in response to your critique, is not any defense of Plantinga's Free Will argument, but a resort to the more common arguments against The Problem of Evil -- IE special pleading that God's "good" is different from and unknowable from our "good" (I. E. -- God is not omnibenevolent), or that God is unable for some reason to optimize the world (God is not omnipotent, or not an agent).
The reason the "Problem of Evil" remains a "problem" is that none of the rationalizations that have been trotted out to try to deflect from it, stand up to scrutiny.