So, I Become A Side Villain Now?

A mercenary soldier from the future with the code Dark Saber is eager to retire soon. However, a dimensional crack disaster occurs on Earth, forcing him to fight against creatures from other dimensions. Despite Earth achieving victory, Dark Saber perishes in the battle. However, later he awakens in a strange place. "Wait, I know this place! It's..." ... He woke up in a game he hadn't played in a long time. And what's worse is, he possesses the body of an insignificant villain who is destined to die as the protagonist's stepping stone.