Akuochi Onna Senshi to Yuusha

Once-good female warrior Rurina has joined the dark side.
After a lover's quarrel she took a stroll outside the demon castle.
That's when she came across Ayta, an aspiring young hero.
"Rurina, it's you! Why? Why did you join the dark side?"
She chuckled. "You came along at just the right time..."

[From DLsite English]