As a somewhat negative preamble:  Plantinga's argument is something that I consider to fall into the realm of apologetics, not valid philosophy.  The goal of apologetics is to provide first order plausible explanations to problems, to allow the faithful to avoid thinking about the problem.  I have little faith in the quality of any defenders of Plantinga, or in Plantinga himself on this question.  

Your critique is entirely valid, and you touch on 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 in not just based on freedom, but also upon inclinations.  One can be entirely free to do nothing but harm, but also have no inclination to DO harm, and therefore which no difficulty choosing not to do harm.  So -- what inclinations do 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.  

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. 

There are several other objections to this argument, which address other flaws in it.  One of those is that his argument presumes that the "good" of free will trumps all the significant evil that it presumes free will requires to therefore happen in the world.  BUT, humans have VERY LIMITED free will.  We have limited knowledge, power, imagination, and ability to control our own inclinations, AND we are highly constrained by the choices of other.  IF free will were SO morally valuable as this argument presumes, then any creator God who left us with such a limited portion of free will, would have created a morally deficient world, compared to one in which we could have had much greater freedom.   

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. 

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.