I don't know... I think both his "bad blood" and his upbringing (not having a parent to show him what love is) turned him into what he became.

Yeah, it certainly contributed, but it was a mixture of things.

I'm not sure if anyone mentioned this yet, but the real reason Voldemort was the way he was is because that's how J.K. Rowling wrote him. That may be a sarcastic answer, but it's absolutely true. As complex as Rowling characters and stories are, real people are much more complex. All of your theories about genetics, upbringing, nature vs. nurture, with cool theories about being born under love potions with Slytherin's blood and growing up in an orphanage are all valid. I bet all of those things are attributes Rowling considered when creating the character too. Those theories and background help make a fictional character more realistic. However, Rowling needed a character to fit the quintessential villain archetype, so Voldemort was created to fill that role. So my answer is that Voldemort is the way he is because he fills an important role in the story of absolute evil, someone completely devoid of love who is on a relentless quest for power. He becomes more real by Rowling's amazing writing, and by our abilities to analyze him, discuss it with friends and places like this, and create a deeper picture in our minds.

I agree...

Voldemort was born evil and will always remain in history evil. Thats all there is to it!