What's wrong with love?
It's a novel anyway, not really life. According to JKR, The ONLY one thing Lord Voldemort doesn't understand is LOVE. Harry's maternal love grants him protection and saved him from Avada Kedavra. And because of love, Snape didn't become a death eater per se. Dark as it is, Harry Potter is a children's book, like old fairy tales, " prince and princess lives happily for ever". Just imagine how kids would behave if Harry dies or if the plot tangles too much conspiracy or other evil.