It is because of our misguided notion that if we support someone who did wrong, even in TV or movie, we are supporting wrong: supporting a negative character shows that your thoughts are negative as well. This common assumption on a subconscious level leads us, mostly people belonging to the region of South, southeast and east asian regions. You move on to Turkish drama, English or American drama, white, grey or black, actors wear their characters with pride and are loved for their characters.
Damon Salvatore from the Vampire Diaries was one of the darkest male leads I have seen in my life and yet I loved him and so did millions of his fans. Elena, who became his girlfriend later on in the series was the typical good girl who hated him for his negativity and yet fell in love with him and it took her time to accept her feelings and deal with them because she wanted to be with him but accepting him for who he was was not so easy for her. This, this is what character development is. Damon is a being,Elena is a being and they have their own strengths and weaknesses, and their own likes and dislikes. How had problems like any normal couple as opposed to Hindi serial couples who never have any conflict regarding even what to have for dinner, let alone have any ideological conflict. They never need to sit down to discuss where and how they are different and how they struggling to be a part of each other's lives. The only times the mushy mushy indian couple fight is when a third person comes between them and purposefully creates a conflict or even if not purposefully or intentionally, the conflict is always the result of a third person. I initially like Yeh Hain Mohabbatein because Raman and Ishita were characters with depth and fought even after being in love because they viewed issues differently and wanted to handle issues differently. I started watching Dil Sambhal Ja Zara again because there is character depth and development. With EDT I expected that depth, non-linear multidimensional characters in Shiv, Vyom and Sharanya and even Rajan, Madhvi...unfortunately it failed to meet my expectation.
Although I divulged from your question...my apologies, I think it is the morality indicator culturally instilled within us that refuses to accept that we support someone negative and we forget to draw the line that the character is negative and not the actor. And also, we have this tendency to think if something or someone is wrong, we need to fix them. We can't handle the idea of loving negative characters. I don't see anything wrong with someone falling for Vyom, the psycho that he is. Yes, the girl's life will be messed up, sure, but that she will realise after falling in love, right? It's time Indian television stop blurring the lines between a mental patient and murderer and start blurring lines between good and bad because most of the time, human beings are between that line: in the grey line.