The Corruption is a cancer caused by the sins of those living on a Titan. The vile actions and thoughts present in all beings feed the growth of the Corruption as it spreads relentlessly across each world. The Corruption knows nothing else but to consume everything it touches, leaving behind terrible creatures of hate that exist for the sole reason to cause pain in punishment for the unearned pleasure experienced by living things. After the Corruption restores balance to life, it destroys it with the goal of turning the world into a desolate abyss void of life.
*small change of being a reference to Terraria