Why kill the parents?
The trope of the dead, or dying, or absent parent hardly started with young adult fiction. It goes back to fairy tales. On its most literal level, the death of a parent--usually at the very beginning of the story--serves to reassure children that there is a story afterwards. That is, even if your worst fear takes place and you are left orphaned and desolate, you will survive, your story will keep unfolding, and you will ultimately triumph. The anxiety around loss and abandonment cuts so deep that this is a story which simply can't be told too many times. And often some form of an internalized parent will go with you: perhaps your dying mother will give you a tiny doll, along with very specific and slightly creepy instructions for its care, and that doll will save you when your long-gone mother cannot. You won't be entirely alone.
But dispensing with the literal parent serves another, darker function. It allows the protagonist's (and the reader's) ambivalence to come out as multiple parental figures: most simply, a good witch and a bad witch, or a fairy godmother and an evil stepmother. Bruno Bettleheim thought that one of the most important things fairy tales could do for children was to allow them to hate their parents safely. The story says, Here are your darkest feelings, the ones you cannot look at directly, the ones you feel will destroy you with corrosive vileness if you fail to suppress them for a single moment: the desire to watch your mother melt away, to see your father topple from the heights of the beanstalk. Wait, you can entertain these desires in the form of a story; that is not really your mother turned into a puddle, just a witch. The story provides a sanctuary for the rage that is intrinsic to all of us. You can have moments of hatred and fury, live through them--and still turn out to be a good person, even a hero.
So, yes, most teenagers have to deal with literal parents and all the issues that come with those parents. They have to negotiate independence and do their homework and cope with the moments when those parents won't act like parents at all. But they also have their non-literal parents, the ones spun of fantasy and darkness and longing: the parents who have always been dead, who are always dying, and who will always live forever; the ones who seem to fragment into multiple figures, some disgusting and some adored; the witches and angels and ghouls. A literal parent is no more the complete reality than a fantastic one; we are all of us, always, at least half dream.