i think that it had a lot to do with his mothers sacrifice as well as his hatred for the dursley's that kept him from turning out badly. dumbledore says to harry that the reason why voldemort cannot stand to posess him is that his mothers love run through his veins (or something along those lines). while, yes it is a fictional character, i dont believe that JK would have written him to be so genuinely kind and loving without there having been a tangible reason in her writing..... idk that is just my train of thought on the subject