i think you are right. and karan didn't enter the show as a negative character. he came as the son who demanded his rights for his mother and himself. that's not considered negative. negative is when you willingly do bad stuff to other people, i.e.: meera and mandira.
as for tanya turning negative, she isn't all that good bahu either. if you notice in Wed's episode, she taunted karan about Tulsi's reaction, which sanskari good bahu does that? and she is already shown being rude to baa and no one is ever rude to baa, especially not a positive character. karan never had been rude to baa, in fact, he only entered the house when baa had come to get him as baa had told him once to never SN (early in karan's initial days). and she never scolds her son for the wrong doings. if she is a good bahu like damini-ganga-nandani and tulsi, then she would have been shown to scold manthan not to give him love and even more assurance that its all right for him to do whatever he wants, just because he has a rich father.
humaira