Baldur's Gate II: Enhanced Edition

Have your main character complete one of the following:

Hero - Simply be good and complete quests of a good nature to improve your reputation level to the maximum level of 20.
Villain - Simply begin killing peasants and neutral characters to lower your reputation level to the lowest level of 1.

Both achievements can be acquired in a single playthrough by saving before completing one path, reloading, and then playing out the other path.