Very well analyzed post Mary đđź
Human behavior is more susceptible to external stimuli, Shlok's behavior has a very strong reason, which you have listed them all .
No one is born evil, it's always the nurture and the environment which builds the character of a person . Shlok is not an outright evil as we all know.. but he has different and darker shades to his character
A household like the Agnihotri's, profoundly stiff and disconnected and disgruntled, with no freedom of expression, freedom to smile or laugh with family members, no space for love, friendship, affection and warmth, attribute to gloomy and dispassionate behavior in kids and as we know that NA, the puppeteer always makes Anjali do the dirty work there by Shlok hates Anjali and worships and hold his dad in a higher pedestal to worship. so he is inhibiting the traits of "never take a NO for anything" being stubborn to the extent of getting what they want either by hook or crook., and being extremely manipulative, to name a few. I am not sure if Shlok will abuse Aastha the way NA does to Anjali .. because logically Shlok is still unaware of this side of his dad ... so him abusing Aastha may not be the case .
And your point regarding Shlok craving for love and relations is very true, which we will see in going further, the kids in such disgruntled family often crave for love, care, trust and relations... and here Aastha has to live every relation to Shlok .be a wife, a sister, a mother, a friend.. in all this Aastha's freedom will be choked.. as in a relation giving space to each other is very important .. and if she threatens to stay away, or demand for her space that is when she will have to face the dire from Shlok . In future I guess, Shlok loves her to an extent that it suffocates her...
His anger and his possessive nature are due to the helplessness of not able to get the needed love, care, affection from the family or elsewhere(swati).. he is getting that from Aastha, who trusts him blindly and who loves him unconditionally .. he would be clinging and holding tight on her emotions ..he wants to be the center of her world and this is when he will not be able to take any rejection from her .
Varad is different, he is highly dissatisfied with his life, but he accepted it and his reason for being the way he is because of his daughter and the mutual love between the siblings, but is he happy ..no, for that matter NO ONE in Agnihotri house is happy
the other important thing which I can add in this topic is nature vs nurture., Nature is what qualities you get from your biological parents, as you said the traits of NA are by nature from the roots.. and Nurture. is the environment, the school, home, siblings and friends (strangely not a single friend of Shlok is shown in the show) holds a key relevance in shaping up one's character. Shlok's character has different shades owing to the various factors in life. but above nature and nurture is one's own "conscience" of what is right and what is wrong . we see the glimpses of it in Shlok every now and then and in future this very conscience of his, will get him out of his darker shades.
I haven't personally seen any such families *touchwood* but read about a lot of such families and kids in US, giving the right atmosphere for kids is the responsibility of parents as the healthy childhood shapes up a person into a good human being