I also think Sarika was important for Vikram to realize his true feelings for Shobha. But Sarika did not have to be this psycho killer for that. We had enough with Samarth being an A grade A@@#$%$# and now his sister is in the same category. It's a bit too much .
In fact, it should have been shown that Vikram realizes when Sarika is back, that it was infatuation and not love, since Sarika and he are not compatible at all and her needs in life are materialistic whereas his are emotional. It should have been a practical realization that it is only Shobha who is his soulmate and Sarika was a past relationship that simply did not work out for a good reason. Why make another character so evil to prove that Vikram and Shobha deserve each other?
Also, it would have been way more real and interesting, if Sarika was not evil, but just a more materialistic person, who just did not love Vikram the same way. And Shobha who would try to set them up, but on the way, realize first that Sarika is not for Vikram and being super concerned about both of them she would feel that it would not be right to set them up. And secondly, she should have realized that setting Vikram up with someone was proving to be tough on her because she realized that Vikram is the one for her and she has fallen for him.
It would be a much more real show then. But people don't like to watch intelligent and real stuff, they want drama and nonsense to entertain. Hence the negative characters.