Jesus might have "rendered all things clean" by what he said, but that is hardly at the heart of his words.  Yet even if we say that he is pointing out that evil comes from our innermost being, our desires and our choices, the question remains: what is the source of that evil in us?  Did God create us that way, and so we are not guilty of what we do?  Or what other answer is there?

I think that the question itself is invalid.  The fact is that evil exists, that it is in us, and that we choose it and spread it to others.  The evil in my heart has been there as part of the legacy that I have received with everything else in my life from my family and my community.  But far more important is the fact that I have chosen to cooperate with evil and so have a fair amount of responsibility for the continuance and even growth of that "non-God" within me, as well as for my spreading it to others in my community.

Jesus is calling me, not to deny the existence of evil in myself, but to struggle against it and to live like a child of God:  no child is perfect, but it gives pleasure to its parents as it struggles to grow beyond that imperfection (whether that is immaturity, incompleteness, or sinfulness).  Just so do we please God as we try to cooperate not with the evil but with his love in order to grow and become perfect in his firstborn, our brother Jesus.