I think Vishwas and Avantika's relationship is less about love and more about the thrill. They give off major naughty vibes. 
Avantika is a highly sexual character. She’s a guest in a conservative house, yet she’s walking around in silky lace nightware, and why are her blouses always off-the-shoulder? Vishwas loves the X-rated aspect of their affair, but they could never have a functional relationship because Avantika is too fierce to ever let him control her.
For Avantika, I think Vishwas was how she secured her position at the hospital, but now it’s about Sanya. Sanya is illegitimate and legally doesn't deserve anything, so Avantika is playing dirty to secure that Tandon money for her daughter. Sanya can only inherit her father's self-acquired property, not the ancestral Tandon wealth. This is exactly why she hates Aarambhi, who has Vihaan. Vihaan is the one legally entitled to the main estate from birth. This is also why Avantika did not care what happened to him during the kidnapping.
I think the hate really started when the Tandons moved. For 10 years, Avantika lived with access to Vishwas whenever she wanted him. Now, she has to endure hearing Aarambhi called "wife," and seeing the sindoor on her. But the moment Avantika was really mad was in the beginning when Aarambhi crashed her and Vishwas’ anniversary. That made her realize she would always be 2nd, and she hates that.
Without all the petty tricks, plans, and schemes by the villains of this show, there is already a great amount of drama. This could turn into a really big legal battle.
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