Why do you assume his/her parents didn't know any better? Some people really are selfish and malicious. You're going out of your way to assume the best when you don't actually have any reason to believe that's true.
There's a general sentiment that all dogs are innocent at heart. If the dog has bad or mean behavior, that's the consequence of its bad experiences, like being outcast as a stray, bad owners, having been hurt before, etc.
You can apply the same line of thinking to humans. Everyone is a victim of their circumatances and experiences.
And while I understand that "everyone is innocent at heart" might be true to only some degree both with humans and dogs (nature vs nurture, etc), you can't really tell by the end result.
Even if they are born that way you can still look upon them with pity. They didn't choose their parents and they didn't choose how the wiring of their brain.
