Very good question Roza...After great consideration and pondering over this question I can only say one thing...
Sue started this evil game by brainwashing Mallika about how had it not been for Nidhi's appearance in ashu's life he would have definitely married her...But the greater part of the blame lies on Mallu's weakness of mind...A weak mind gets easily misguided and the person begins to think that whatever he/she is doing is correct...
I had read in psychology/criminology that every human being has a dark side to him/her along with a good side...In some the dark side overtakes the good side and the person does all wrong things...But to him/her whatever they do is not wrong...They are so obsessed with what they want to achieve that they will use any method to achieve what they want...As we say in Hindi...'Saam-Daam-Dand-Bhed'...Thats what Sue drummed into Mallu's mind and now there is no going back for Mallu...
As we say Hell hath no fury like a woman scorned...And Mallu has been rejected time and time again by Ashu...And all because of Nidhi...Or so Mallu believes in her sick mind...
In my opinion the greater part of the blame should be given to Ashu...The moment he realised that Mallu loved him...and after Mallu herself admitted that she had done so many things to separate him and Nidhi, he should have wised up and drawn a bit away from Mallu...Saying I don't love u and doing things which indicate feelings to the contrary would have made Mallu more convinced that were it not for Nidhi Ashu would have been hers...And so far since Mallu's return Ashu has always chided Nidhi and that too in front of Mallu...
Conclusion: As it is extremely difficult to decide who is to blame I thought of one name...KP..He is the brains behind this track so he is to blame...😆...Sorry for such a poor joke...