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Yeah, the kidnapper is definitely needed for Do Bong Soon's growth to an actual hero.  Right now she's fully capable of handling bullies and gangsters, but as we have seen when it comes to true evil her emotions tend to take over and she becomes weak.  In order to confront the kidnapper she's going to have become the hero she so wants to create in her games.
He's needed to make bong soon gain more confidence in her power i think. but i do think that the serial killer is a bit too scary for this typr of drama. So much gore in his scenes.. yuck

I think it is necessary, so that DBS can flaunt and use her super strength throughout the series. And I love the part when the 3 leads team up together to defeat the main villain, that was so cool.  The writers could have explore more of that. They could have made the plot line more smoother, more compact, instead of throwing a lot action challenges to DBS in all direction.