The Billionaire's Detective
He realizes it's more than just a coincidence — someone is doing it, and he needs to stop it.
When he brings in the third lady, she isn't killed immediately like the rest, but after a month the lady is killed. All this while he never suspects Bella because he feels it's his opponents doing it just to get him down.
After that he brings in a fifth one as a bait. He has his men watch over her apartment day and night, and no one comes. They don't approach her home; they just watch from a distance, until three days pass and she hasn't shown up outside. They decide to go in and see for themselves, only to find out she has been dead for days.