Takeuchi: That's why Rider was evading him. As with Saber, Rider had a way to kill Berserker for sure in Bellerophon, but she knew that Berserker would kill her as soon as she killed him. Because of this, simply killing Berserker once or twice would not be enough to ensure true victory for Rider.

Fate/complete material II; Characters: Berserker, pg. 64-65