I think Ruj going after Saimai was a mistake born out of obsession. He's friends with her parents and had been seeing her since forever. He probably couldn't "hold himself back" anymore, and went after her. I think this because targeting her seems weird compared with all the caution he has while raping the girls. Saimai is way to visible and a risk, that's why he retorted to threaten her with other girls' videos.
The thing is... Ruj is still stupid for seeking his targets on his workplace. Perhaps he thought no one would believe "tainted" girls?