48. It's the only way to undo something horrible that happened to the BBEG. For whatever reason, a consequence of his scheme results in the world's destruction. This can take on many different flavors, from a "suffering" BBEG whose efforts are actually noble but misguided, to a greedy villain who will stop at nothing to regain the magical power stripped from him in the past.
49. The BBEG is being coerced/forced into it by a greater entity. (Usually some mighty agent of destruction --a demon, a Far Realm entity, you know the type). This is a way to include the typical "Exists purely to end existence" type of entity while adding some flavor to the situation.
Probably the most effective antagonist I've ever used was a combination of those two. He was a tortured cultist warlord whose attempts to withdraw from the cult led to the death of his young daughter (the only person he loved --a notable distinction, given that he didn't even value his own life). Due to her soul being held prisoner by the demonic entity that slew her, the BBEG was forced to work tirelessly to break its prison in order to save his innocent child. He didn't do it for himself, and while he ultimately died trying to save her, his plight really struck a chord with the players, and helped enhance their hatred for the demonic entity.
49. The BBEG is being coerced/forced into it by a greater entity. (Usually some mighty agent of destruction --a demon, a Far Realm entity, you know the type). This is a way to include the typical "Exists purely to end existence" type of entity while adding some flavor to the situation.
Probably the most effective antagonist I've ever used was a combination of those two. He was a tortured cultist warlord whose attempts to withdraw from the cult led to the death of his young daughter (the only person he loved --a notable distinction, given that he didn't even value his own life). Due to her soul being held prisoner by the demonic entity that slew her, the BBEG was forced to work tirelessly to break its prison in order to save his innocent child. He didn't do it for himself, and while he ultimately died trying to save her, his plight really struck a chord with the players, and helped enhance their hatred for the demonic entity.