Do you mean God? This is certainly one of the options, but before we mention absolute powers, I'd prefer to find out, for example, the odds of a man changing his own nature himself (and circumstances, in which these odds are highest).
In a reputation comment (is there a way to reply to these directly? Pretty embarassing to admit, I'm CS major...), rawr has said "trauma, extreme stress, drastic change in life environment" which are all valid points when discussing major changes in a person's behaviour, but then:
1. I probably should have been more clear about what this "nature of man" is; and that's actually another big question: What defines the nature of a man? What does this nature consist of? Is it something inherent, defined clearly and pretty much unchangable in a way other than destruction and complete redefinition? Or is it something that is actually designed to be changing all the time (vide Sartre's Being and Nothingness)?
2. Trauma, stress etc. by definition affect person in a strongly negative way. Putting aside the question stated in (1), are there some positive circumstances that may change a person? Or is it actually pointless to talk about positive occurrences per se, because they exist only as a contradiction to negative ones and are temporary in nature (loosely following Schopenhauer)?
I do think there is such a thing as human nature, I think its a really rich body of research and thinking that exists on that topic too and well worth reading, if only to disagree with some of them. Broadly, among the best, I think are the views of Erich Fromm, Karen Horney and Bowlby and the other attachment theorists.
From Fromm I take the idea that man is naturally productive, naturally relates to others, is reasoning and loving but when these drives are blocked they become distorted or perverted, into various pathologies, although never cease. I think there are two over arching personality types, biophilious and necrophilious, people begin as biophilious and it could perhaps be considered natural but in so far as necrophilious personalities are also human (all to human, perhaps) it could be considered natural too (Fromm believed in a kind of humanist maxim that "nothing human should be foreign to you"). Finally I believe also that mankind has two modes of life available to itself, being or having. I think Fromm theorising about social character, how the demands of society and economy shape individual character are worthwhile too. Sociology is as important as psychology in shaping his perspective and I think that is very important.
From Horney I think the theories about basic anxiety in the home giving rise to neurotic trends, moving away from people, moving towards people, moving against people (aversion, obsession, aggression) is well reasoned and corresponds to some of my experience of others too. It corresponds to some of Bowlby's and other attachment theories thinking too.
From attachment theories the idea of the importance of attachment figures, primary carers, to forming internal working models and attachment styles which can be avoidant, ambivalent or secure (organised) or disorganised, is important. That these styles are rewritable but pretty constant I believe is true too.
So I believe there is a, debatable, human nature in the broadest possible sense, Fromm has the most to say about this and also that society is out of step with it but he takes it as a hopeful sign that man is not infinitely adaptable, he does not conform to the present mode and demand of society and the economy and can not and will not forever, eventually entropy and crisis will demand he return to himself. In the most individual sense I believe that attachment style and basic anxiety or neurotic trends are significant. Internal and external factors are deterministic of an individual's nature, they interact dynamically.
Trauma will change people, it can have some permanence depending upon their resilence or lack there of (determined by attachment style, adaptability, coping strategies), do positive things change people for the better? Perhaps, although I would see that more in the sense of opportunities or conditions for change and that change I would reason is the recovery from trauma or its legacies or some kind of social character which does not correspond to human nature in the broadest possible sense of the word.
