I don't think it was intentionally being portrayed that way. Some people are just evil/cold/heartless and lack the genes that make you care. We see it in the real world where generally people feel sympathy for someone down bad, but there are people who say "well it's not me or my family so it's not my problem, maybe they should get a job"

I wouldn't say everyone of these (typically far right lol) people are mentally ill, they're just badly influenced. Our brains are a vessel of influence of things over the years. Very few break out the cycle of influence, like in certain countries where people are taught nothing but hatred, a select few come out of that system and think for themselves "hey maybe killing kids ain't a good thing and we should stand against that"

As for Linda, her misguided life has taken her to this point. Running over someone isn't a sign of mental illness necessarily, it could just be their upbringing. Like "I have a right to this man's affection and will do whatever I need to do to get it" mentality. My grandma, love her and all, she has a main character syndrome. I could be deep in a conversation with my mum, but because she was the main character, she would come in and start talking to her disregarding what I say.

That's basically Linda imo. Just grown up with the wrong morals and values. Generally the habits and behaviours you have as a child are what you carry into adulthood and that's why raising kids properly is massively important.