I would say 'trust' is a two-way street. Nandini's trust for Manik didn't come in a day, nor was it unexplained in any way. It came because he had 'earned' it. Given their history, Nandini could have chosen to ignore all the little signs of goodness she had seen in him and walk away, but she did not. Willingly or unwillingly, he showed his trustworthy side to her through little incidents, and she actively chose to believe in him and overlook that 'monstrous' side that she was well-aware existed in him.
Trusting someone is always a choice you make based upon their actions. Expecting Nandini, or any other character for that matter, to be a mind-reader and get through immense pain and shock in order to read his eyes is extremely unrealistic in my opinion. Yes, unconditional trust should exist in a relationship, but that can only happen when it is supported by communication. Every time Nandini 'saw through' Manik and comforted him, she already had an inkling that he was disturbed in some way, be it when he hugged her or when he shouted at her when she told him about Cabir. This is why the responsibility for this particular fiasco rests solely on Manik for me. He simply did not communicate. Instead he deliberately hit her where it would hurt her the most. He cheapened the trust he had built himself. Nandini, in the future, might figure out his reasons. I am hoping she will. But her 'trust' should not return until Manik learns to communicate honestly.
@bold: I don't quite agree...firstly, no one asked Nandini to be a mind reader...but if trust was claimed, one would expect it to display itself in a crucial moment...Nandini gave him her trust even before he really asked for it...the morning after the drunk scenes also he was showing her his "evil" side by insinuating stuff but she didn't believe him then and chose to go with what her heart told her.
This incident was much worse no doubt and Manik was at his evil best...so it was tougher for Nandu to look past that...but that being said, I still feel she believed the worst from him a little too quickly...like somewhere inside her even she didn't believe this was for real...that there had to be a catch...and when Manik gave her half a chance to believe that yes it was too good to be true, she fell for it.
So no, I don't think the blame rests solely on Manik here...a large part of the blame, sure...but not all of it...Nandu didn't need to be a mind reader to go with the instinct that she's gone with so far with regards to him...whether it was the time he needed that comforting hug from her or many other incidents.
Circumstances, their youth...I blame these things more...they are too young to really handle this well at this point...they are both making mistakes...those mistakes should teach them something about love and life in the long run.