This quote by Margaret Mead means that human nature has both good and bad in it. I would think that most people would agree with this if they simply look at the way our world is and the role we humans play in that world.
It seems clear that human nature can be aggressive and destructive. People act aggressively towards one another all the time. We rob, murder, and rape. We abuse people who we are supposed to love. Our countries engage in wars against one another. It is very hard to argue that our nature is not, at least in part, aggressive and destructive when you look at how likely we are to harm others.
At the same time, it is hard to argue that we are only aggressive and destructive. Instead, it seems like we also have an innate tendency towards being orderly and constructive. Most people obey most laws. We do not typically need police to stop us from running red lights, or from stealing from our neighbors. If you go to an amusement park, you see people waiting patiently in line rather than trying to push in front of one another. We are also constructive. We are constantly trying to make our world better. Teachers work hard to help students learn. People like Bill Gates give huge amounts of money to help people get educated or to alleviate problems in remote parts of the world. This, too, is part of human nature.
What Mead is saying, then, is that we people have good and bad in us. Part of our nature is aggressive and destructive, but human nature also makes us tend to obey laws, to be good to others, and to try to improve our society and our world.