It was as I guessed it.  Yeon Joo originally was the one who drew Kang Chul character which her father took her inspiration and developed Kang Chul character further into the W comic.  So she intellectually gave birth to him and her father raised him with infusion of his own yearnings to be successful and talented.  
What does this mean?  That Yeon Joo fell in love with an image she gave birth to in her mind which was given character by her father. It is the same as falling in love with an idea she created.  Falling in love with the deepest yearnings that her father wanted to have that was expressed as Kang Chul success of fame and fortune.  
How many of us fall in love with an idea of someone instead of the real person, only to find that reality is nor exactly as we thought?  Yeon Joo sees Kang Chul as the good guy, handsome, rich, driven, talented, but there is something more that she is now encountering:  a free will character that is unpredictable and morally ambiguous. Not exactly as her father and her imagined him to be.  He is breaking out of the mold of model character of righteous values who abides by the law and morality which Yeon Joo sees.  Not exactly the idea that she fell in love with!?  I wonder where or how the writer can develop things from here?   It's one thing to get shot and not be hurt, accepting an explanation of Kang Chul saying that he knew she cannot be hurt,  it's another to know that he intentionally shot and hurt her father.  How can love develop from here?   I would hate Kang Chul if I was Yeon Chul, much less have a love/ romance relationship with him?!
So falling in love with an idea is not the same as having to face reality that the man you thought fell into predictable mold is really unpredictable....hmmm where do we go from here in a way that would make sense?
@Lotus There's a episode as well, where it shows young Yeon Joo drawing a character which is highly likely to be Kang Chul. Based on this episode, it's clear to say that the Seong Moo her father discovered her drawing of Kang Chul and decided to develop the character further with his own ambitions in mind.
@Lotus, I also think of it this way (if you want to view it in a romantic perspective) --> KC was created by YJ. He's everything that YJ wanted in a guy so in a sense KC is made for her. The only difference is, her imagination of a perfect guy is just an idea. Her dad created a world for KC to live in and possiblyfor his daughter to have something to look forward to (to read). The only thing different now is that KC the fictional character is no longer fictional...he is very much a real human with free will of his own. This makes him human and no longer just an idea she created. I guess that's why she can justify falling in love with for real.
Sorry for bad English. @Bongsookie I do not think the passion Yeon Joo is the man she had created, as who did the personality traits of Kang Chul was her father. She created a few ideas through drawings about what kind of man Kang Chul would be at most a child can create. In terms of character creation suffered by Yeon Joo, she drew as a way to escape the terror that a child suffers when their parents fight.
In the same context, I think the killer was also created by Yeon Joo, who is the personification of evil and the suffering of her family.
Now we know even Kang Chul's looks and personality are created by YJ, just as I suspected.  He is her ideal man, she can't help it falling heads over heels for him because he is everything she wanted (minus the shooting her and her dad).

I am also suspecting that the killer is the darker side that YJ created.  When a child is caught between the conflicts with parents who are agressive and fighting with each other,the child feels abandoned and lonely and wished that the family members go away or not be there anymore. This is probably the reason why the killer goes after all of KC love ones and family.  
I came across this on Tumbler...it is such a wonderful insight into Kang Chul and Yeon Joo's sentiment during the police station scene in episode 7.  I can't help it but want to share this :

persephunee:
You know what I breaks my heart about Kang Chul? Up until Yeon Joo confesses her love, he literally cannot comprehend why she, or anybody would want to save him. He hates himself, he thinks he’s a monster, and he doesn’t think he deserves to live, let alone deserving to be saved. He doesn’t believe that he deserves love even though he so desperately craves it and needs it.  He’s also so angry because he feels so betrayed. He wanted to die. He wanted to end everything, and it was the one thing he had control over. He thought he had finally gained a semblance of free will, of choice, when he jumped off that bridge and yet again it’s taken away from him. He’s constantly treated like a fictional character, like he’s not alive, human. Nobody ever asks what he wants, and when he makes a choice it’s immediately taken away. Can you imagine being so self aware, and yet being stripped of your free will?  “Am I some kind of toy?” he says. Why would you hold onto such a pathetic webtoon…?”Because why should Chul be saved when he has nothing left. EXACTLY. He thinks he has nothing left because he is just a creation. But what he doesn’t understand is that he has created his own personality and his own destiny and that makes him something – SOMEONE – worth saving. He means something to Yeonjoo so he is human. What are we if we are not people defined by what we mean to others? Honestly this whole scene broke my heart. I don’t care if people don’t think ‘W’ is Jongsuk’s best acting; this scene is brilliant and his quiet fury lights the scene up.  That extreme loneliness that he feels already is quadrupled to the infinite because not even his friends are real, not even his family was ever real. They don’t have days when he is not there, the radio show doesn’t broadcast any news that is not relevant to him. The police obeys him because he is the main character, he owns riches because someone wanted to draw fancy apartments to satisfy his own aesthetic greed. He owns nothing. He is no one but a figment of fantasy. Now this girl tells him that she wants him to be something else – what else can he think but that she wants him to satisfy her, but only in her own way? He thinks she wants romance to make up for the lack of it in her life, he thinks she doesn’t want him or even care who he is, she will just make him into some Jack Sue character. He turns from a black-suit revenge-seeking killer, into a beige-cardigan wearing romantic hero. But this is still not him. There will never be him. He can never escape. What about for her? Her heart is shattered too because what he is saying is not wrong… she did create him to fall in love with when she was young. But the Chul she loves now is the real one – she really loves him but feels that she may have done something irrevocable to hurt him. It just hurts so good – a scene where both of them come from places that make sense but you really want them to get over it and be happy together.