From the placement of the end of this chapter, we can infer some things (I dont see why else chapter would end here directly after talk of selling her): her first customer (mc) was somewhat young, and thus she fancied him a little. Then after he had brutally had his way with her she latched onto that feeling, deciding on it being love and that she had given herself to someone she loved. Then her obsession made him bring her with him to exploit her blind devotion and fear of abandonment, turning her into an assassin helper. I bet he trains her in 'counter-interrogation' a lot too.
Any other option is just impossible. Like her being saved by mc? Yeah, no, how could that happen? That isn't assuming-the-worst-instead-of-the-obvious-cliche-to-take-out-your-annoyance-with-the-manga-on-the-manga, so I don't see that ever happening.