I welcome such a discussion and find this question enjoyable, especially because it merits personal views and not just carved-in-stone texts.
Even from a scientific point of view, defining "what is nature (natural)?" has been a difficult task that has been evolved in different fields such as meta-physics, astronomy, mathematics and religious studies, etc for many many years.
A simple question as follows could solicit long discussion, which could bring up hundreds of other questions that would eventually become a "chicken or egg?" problem:
Is acid rain natural?
If one thinks in her own logical way that has been taught in school, the answer is Yes and No.
Some say it is natural in a sense that it is "cause and effect".
While some say it is not, because it shouldn't exist.
Putting science aside and leaning back on my own personal view of what I have read/learnt in Buddhist books and articles over the years:
One thing we could be sure of:
We are all born with a good heart and will. Having a good heart and will not in a sense that we start making donations to charities, look for someone to save, or counter others whose deeds we deem "bad".
Rather, we were all born a "Buddha". But we are not (yet), due to "senses (feelings)" that we so quickly acquire the second after birth.
It tastes good.
He is handsome.
She looks beautiful.
I must do it against all odds to uphold and thus protect this, because of my tradition. (clinging)
I hate ....
It is so funny (laughters).
From my own thinking based on books that I have read, people and nature that I have met and observed (including observing my own self, too), to be "natural" (nature) is to lose those layers of "cocoons" that bar us from being a Buddha since birth.
Those layers being - senses and feeling - hence thoughts (judgements, logic, biases, preferences, liking, disliking, stubbornness, heroism, etc etc).
Other words, following the buddhist path is to become one with everything and all (even an ant is of greatest "importance" in its own existence in this space).
The preceding on "Nature" is only my personal view based on my own experience and thinking.
The following brings us to "Behavior":
Behavior, in my opinion, is what has been mentioned above - senses and feelings - all these surmises to thoughts.
Thoughts bind you to do or not do something regardless whether or not it is "right" or "wrong" or "good" or "bad".
It becomes a fuel that "supports" your actions based on senses and feelings.
Other words, behaviors are learnt since brith though sense, feeling - thus thoughts.
"My mother told me...."
"I was trying to be nice to her, but she, nonetheless, refuses to go out with me. I guess all girl with short hair are mean."
"It feels so good, even feels better in the winter....." (senses)
"I loved him so much, although I knew he has wronged me. But I just couldn't help it, I hope he will love me one day and sees finally what I have been doing to myself out of love of him.."
All of the above behaviors could be unlearnt and thus undone, usually though:
"My mother was wrong; now I know because ..."
"No, not all girls with short hair are mean; my neighbor has short hair and she is very friendly"
"You know what, I have read that even it feels good, one shouldn't do it because ......"
"I guess I was naïve; he was not for me, now I met the greatest man of my life, who cares for me......"
The above are examples of "counter-behaviors".
In buddhist sense, the above is not the way we should lead ourselves to.
We shouldn't need to find a counter-behavior to eliminate a behavior.
Other words, we should not pick up the behavior from the beginning.
One does action (behavior) A.
Years later, one finds that action (behavior) A is bad, then she finds (learns) action B to counter action A.
1 + (-1) = 0
It would stay so, as long as action B "prevails"; if one day, through time and experiences, you find action B unpleasant or you simply lose your will to hold on to it, you revert back to carrying out action A.
In short, Buddhism teaches us to not pick up "action A" or behaviors through senses and feelings in the first place, thus eliminating the "need" to find a counter-action such as action B, because the process is clinging and un-clinging, which disturbs our peace of mind and our "Nature" to become one with everything.