You know, it wasn't Maithili's fault. Nor was it Abhay's or Sid's. Just ill luck and fate. For so many years she must have been tortured by inner turmoil, never knowing whether to love Abhay or hate him. Moreover even if she could decide she couldn't do anything about it. When she found out about Pia and Abhay, the jealousy, the hate and the love must've burnt her. To know that who you love has fallen in love with someone who looks like you and is taking your place would've been terrible. He didn't know she was alive but she did and it must've driven her mad. Once she was back in the picture, she must've thought that he'd come back to her or maybe she wanted to hurt him for not being there for her. His rejection made her insane and now more than love it was her stubborn demand and obsession to own what was once her. And i don't think she's really one to blame. She doesn't deserve the hatred but more importance.
Just my opinion. :)