There are a few ways to create the ultimate evil for your DnD (or other media) game!
Choose from the prototypes of any popular mass media for the skeleton to build your villain off of!
The common tropes in media range from Evil boss to Mass-murdering maniac. The general tropes for a simple villain include a sad backstory, a tendency for murder, and a longing to take over the world by violent means.
The way to impart your generic villain’s actions is to really expand on the scope of their evil deeds. Don’t just mention the widespread destruction, but show it and its aftereffects. Have the village that your players were going to head to be torn apart, have the most trusted villagers mention their hatred of the Big Bad, and how the villain affected them and their livelihoods.