This kind of argument only have one answer... no matter how good a parent is, they are not mind reader. They couldn't possibly know all of their kids problem, step or not. A lot of problem can actually be solved with a proper communication. He should have speak any dissatisfaction he might have to them. It might sounds very simple, but the fact there are a lot of such problem meaning the act of talking your hearts out is a very difficult thing.
In the end though, as the backstory potrays, he didn't actually become evil until he was force-fed the parasite thing.
I knew it was going to be controversial and badly recived...but to answer you, I never thought about parents needing to be mind readers.
But if your kid keeps making mischiefs and you keep only scolding them, then you're not a good parent in my book. Also, you don't need to have a phd in child psycology to understand that, if the kid behaves like that, there might be something behind it.
Sure, the reason he became evil it's because of the demon geezer and his thingy, but his obsession comes from before that. It's what the demon exploited.
And I'm also not saying that they never loved him, quite the opposite is clear. But I personally think they lacked the "iron fist" that parents need to use to make their kids sensible human beings (and before I get bombarded with negative reactions again, no I'm not talking about beatings, but disciplinary punishments if necessary and explainations to what the kid did wrong and why it's wrong)