batman affirms and upholds the necessity of trust and integrity for the flourishing of society. why does joker reject this?

The Joker rejects the "fair" society because he thinks it is insincere or fake. He thinks that order is not the cornerstone of society rather than chaos. Joker feels humanity shows its true colors in time of chaos. Joker creates chaotic situations just to watch the world burn. He knows that "when the chips are down" most people will betray and destroy each other: to the Joker, this is human nature.