I think that portrait wala who looks like rithik is actually rithik from his past life.
And colorstv official twitter said reincarnation.. So it's clear rithik has rebirth and i think in his past life he loved shesha's beauty but shesha loved only her husband (naag) then ritik killed her naag and shesha did tapasya to be icchadhari again and take revenge or gave him shraap that he will be reborn and she'll kill him.
Even shesha said.. History will repeat itself but with other ending.
What do you think guys? I cant think of other reason for her revenge
And btw Shesha is the best Naagin ever, even sri devi wasn't so good.. Adaa's eyes and expressions are the best 😊