Who is the main villian in Freak the Mighty and was their punishment justified?
In Freak the Mighty the main conflict is a variation of the classical Man against Circumstances in which the protagonist(s) struggle against the circumstances of life. There is also an element of the conflict of Man against Society in a secondary capacity. In such conflicts as these, there is no primary antagonist/villain. The antagonist/villain is the circumstance of life and/or society that battles against the protagonist(s).
In this view, Kevin can be seen to be victorious over circumstances because, even though life's circumstances ultimately take his life from him, he lives courageously and with imaginative enthusiasm while giving Max a gift of his own zest for being that will be carried on after his own life is ultimately defeated. Also, Max (the second protagonist) is seen in this view as being victorious because, even though his mourning for Kevin is violent and deep and debilitating for a time, he rises like the Phoenix from the ashes and not only shapes his own life anew but puts it all into words to give others the vision of life that Kevin had and had bestowed on Max. In this view, the villain, the circumstances of life, gets off scot-free, as always, with no punishment whatsoever.
Having said this, Freak the Mighty can also be read with Max's father in the primary antagonist role, rendering a conflict of Man against Man. In this view, Kenny's punishment is fully justified as he gets something of what he deserves. There are those who would agree that his was also adequate punishment. On the other hand, there are those who think that crimes against children and youths never get the full punishment they deserve in today's society; so while his punishment was justified, it may have been inadequate.