One commonality between all people throughout history is living in a society. And we didn't need the Joker to tell us that. 

For example, one history meme in this gallery depicts Napoleon after finding out that his wife cheated on him. In the meme, the French general then becomes like the Joker, proclaiming in French that "we live in a society." The implication is, of course, that this influenced him to become the militaristic individual he was. Whether or not this is true, the infidelity is thought to have had a significant impact on him.

In actuality, the affair between Joséphine and the "handsome" Hussar lieutenant Hippolyte Charles began while Napoleon was already away in Italy fighting and winning against Austrian forces. Understandably, news of her infidelity made him "infuriated, and his love for her changed entirely," according to Wikipedia. 

I would argue then, that Napoleon was destined for violent conquest well before he met the once widowed Joséphine. To learn Napoleon's true love story, however, you must also examine his first love to the future queen of Sweden and his brother's wife's sister Desiree, but that's another story. In short, we've all always lived in a society. Now enjoy these history memes.