There are several possible interpretations that you can take from the definitions of matter, principle or maybe even 'of'. Unfortunately in this case, I would have to go over practically every given definition of those words from the 1828 edition of The American Dictionary of the English Language by Noah Webster in order to fully or maybe even adequately explain. That would be much too verbose for this format, even by my standards. I believe these entries are very much worth reading for understanding the matter in detail.
Regardless of that, I can abridge it into one of two general categories:
It can mean that the cause of his nastiness is not caused by nor the is subject of any specific trait; conformity to a general truth; intrinsic human nature or rule. This mostly reassures us of what he said earlier by suggesting his nastiness is an irregularity.
It can also mean that he is not intrinsically evil. The constituent matter of his personality does not originate from malice or at the very least, malice is not the fundamental constituent of his being. His nastiness is just an impurity of the soul.
Either way, it illustrates that there is nothing preventing him from being good