...I would assert that there is no set-in-stone human nature.
Anthropological studies show that humans respond with incredible plasticity to the social and environmental forces around us. As biological anthropologist and blogger Patrick Clarkin aptly puts it, our abilities for cooperation and conflict, love and hate are "triggered under given circumstances, facilitated or hampered by social conditions and structures."
For me, this immediately brought to mind a passage from chapter 5 of Zhuangzi:
"Can a person really have no nature, asked Hui Tzu of Chuang Tzu.
"Yes," replied Chuang Tzu.
"But if you have no nature, how can you be called human?"
"Tao gives you shape and heaven gives you form, so why can't you be called human?"
"But if you're called human, how can you have no nature?"
"'Yes this' and 'no that' - that's what I call human nature," replied Chuang Tzu. "Not mangling yourself with 'good' and 'bad' - that's what I call no nature. Instead of struggling to improve on life, you simply abide in occurrence appearing of itself."
As usual, the above in the Hinton translation and there are some word choices that could be challenged, but I don't think those choices disrupt the general idea here.
Hinton uses the words "nature" and "human nature" for the character 情, qing, as in Hui Tzu's opening question: 人故無情乎. More common translations are: "feeling;" "emotion;" "passion." Thus, Legge goes with: "Can a man indeed be without desires and passions?" And Graham settles on: "Can a man really be without the essentials of man?" Although "human nature" is usually reserved for translations of xing - 性; or ren xing - 人 性; I think Hinton's use of it here for qing is not off the mark.
My sense of what Zhuangzi is talking about here is that he is rejecting the idea of an essentialist, continual, settled quality to human behavior, a human nature of sorts. Our lives need not be determined by some a priori quality of goodness or badness, or passions or desires. Rather we can and should simply respond to circumstances as they arise: "appearance occurring of itself" - ziran - 自然.
Or, as King says: "...humans respond with incredible plasticity to the social and environmental forces around us."
Of course, Mencius and Xunxi would have something different to say on the topic....