One of the first subseries that Lucasfilm produced, the novels were much grittier in tone and more explicitly aimed at an exclusively adult audience, which sharply contrasts with the current subseries, Star Wars: The High Republic, which targets multiple age ranges. The Yuuzhan Vong were the main enemies in the Star Wars: New Jedi Order subseries. In fact, as Reddit user ProfessorLaser explains in a post on r/FanTheories, the Sith Lord could've created his weapons to contain a greater threat: the Yuuzhan Vong.
However, in Star Wars: Legends, there's a theory that Palpatine actually created the Death Stars for a different purpose. This desire led to his creation of both Death Stars in the Original Trilogy and of Starkiller Base and his fleet of planet-destroying Star Destroyers in the sequels. In Star Wars canon, it's abundantly clear that Emperor Sheev Palpatine was motivated by a desire for power, both over the galaxy and life and death itself.
