Most of the reviews here are really negative because of the sad ending.
But, for me the drama was well-written. It depicts the reality of someone struggling for their goals, be it done in illegal way or the appropriate one. I also think some of the viewers don't encounter a tragic drama yet.
They wanted a happy ending for the male lead. In what way could he get a happy ending when day by day he's getting greedier and greedier. He's on his way on top, he's business is doing well, but why would he pull tricks on big companies like that? He's blinded by his revenge towards Jaehyuk when Jaehyuk wasn't interested in helping him coz he's also under in his dad's scrutiny. The main lead got arrogant , even deciding something for Ga young without asking her first. He didn't let her hone her skills in a big company like that and instead considered her working there as betrayal. He walked a rough life since childhood, so I can understand his rude behavior. Due to his worst behavior he gains a lot of conflicts with everyone he meets, so being killed was a possible ending for him. He's lost when Gayoung was able to stand apart from him and it seems he doesn't have any reason to move forward. It's only a pity that he died. On the other hand, Jaehyuk is consistent with his feelings towards the girl. Though, he knows she love someone else. He supported her entirely and not ashamed about who she is, knowing that she didn't hold any degree in fashion. He even told her the location of Young gul in New York but she's adamant not to meet him. His love towards her never falter and she could shine when she's with him. He helps her grow and he believes in her talent in designing. That's the reason why I think she chose him in the end. He sees her worth and thy suited each other.
I believe the writers wanted a story that bounds with what happens in real life. In understanding a story like this, it's best to put ourselves in their shoes and I know we we will understand it's meaning.