Harry Potter is a story written for children so J.K. Rowling didn't spend too much time making realistic characters, plus it's a fantasy series so realism can detract from the magic and wonder of it all.
But let's face it- if Harry Potter had actually survived the killing curse as a baby he would be one jacked up looking kid!
Serverus Snape was a slippery character with a dark past, and even though he turned out to be a double agent in the end all the bad stuff he did while working for Voldemort would look pretty bad on his resume.
And wizards may have figured out how to remove the horrible effects inbreeding has on their children, but in real life whenever people talk about their "pure bloodline" we start imagining what their mutant kids look like. (Comic by Andy Kluthe and Andrew Bridgman)