Man is evil..God is the restraining force that keeps man from being as evil as he could be connect..
It is God's general grace that restrains the evil in the world and allows men to do what other men call good
.Mat 15:19 For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies:
Man has a sinful nature. Man is not absolutely evil in all that he does, something which you appear to agree about. That being the case, certainly you must agree with the concept of Natural Law. If man is not completely and utterly evil in all that he does, there obviously must be a free will to some degree. If there was no free will, there could be no evil, for how would one recognize that an act was evil if there was no Natural Law? It is not just the Christian who can recognize good and evil, is it?
As for your quote from Matthew, it does not say that only evil comes from the heart of man, but only that it is the source of evil. It can also be the dource of good deeds as well, can't it? If you disagree, then you would have to claim that a non-Christian is completely and absolutely evil, a very difficult argument to make in light of reality and scripture.