Shagun isn't evil though. She's just very selfish and her selfishness causes people to look at her in a bad lens.
I see Shagun as one of those self centred people. They have many dreams (often involving material/ superficial things) and are very much concerned with themselves. I'm assuming that she used to dream about having a rich house, hot husband, tons of money, etc. Normal dreams that quite a few girls dream about I think. And then she got married to Raman. Before his change, Raman was a nerdy guy. As appearances mattered to Shagun a lot, his look was not appealing. And not only that, she felt that her dreams were not coming true, she did not have a rich house, hot husband, tons of money, etc. She was strictly middle class and she hated it. She did the traditional have kids, live with in-laws thing. But as a self centred person who thought only about herself most of the time, she did not find fulfilment in her current life. Honestly, I think its something quite a few women go through (especially in arrange marriages). Anyways, then comes along this guy who made out of the stuff of her dreams and it turns out, he's interested in her! Suddenly, it's like her dreams are coming true and she feels more fulfilled. So naturally, she leaves Raman and gets with Ashok.
Regarding her kids, she isn't an evil mother. But as she's selfish, her first priority is herself. Her happiness at the cost of others. So she left Ruhi behind, got Aditya and sent him to a boarding school as per Ashok's wish. But she still does care for them. She likes Ruhi. But not as much as she likes her rich life with Ashok and her happiness. She just has different priorities.
Regarding Raman, yes she cheated him. And I think she does somewhere feel some tiny bit of guilt. That's why she wasn't too pleased when Ashok was being super touchy with her in front of Raman. But again, it doesn't effect her directly so she doesn't give it too much importance.
Honestly, she comes across as a very selfish, but believable person. You can totally see the line of thinking in her brain. All her brain thinks about is herself. And occasionally Ashok because he's become an important part of her happiness, thus self (he provides the money to fulfil her dreams and without him she is nothing and she loves him, physically at least).
I do feel pity for her in that Ashok really is not the perfect, dreamy guy. He's just using her and will probably discard her the moment she gets old or something. But at the same time, she consciously choose to follow what she thought would bring her happiness.
I think since this is an Ekta serial. There will come a point in time where she'll end up repenting and become a typical villain bent on destroying IshRa marriage.