I don’t think anyone was born evil. In Ted’s case I’d say it’s a mix of being predisposed to it and his upbringing. I strongly believe his childhood had an emotional lasting impact on him. Let’s keep in mind that he was away from his mother the first three months of his life and once home she played the part of his sister to hide the fact she was a single mother (more detailed answer about this part here). During that time, he was growing up in an abusive household led by his grandfather, Sam Cowell. He also lacked emotional support. His family wasn’t the kind of family you could share your thoughts or your problems with. Best way to avoid a problem is to ignore it, and it can’t go well with the absence of constraints. As a child and teenager he was free to do whatever he wanted and his mother preferred to turn the other way from some possible worrying behavior during his childhood and teenage years like the knives accident, or being picked up for suspicion of auto theft and burglary. He also started living in his fantasy world at a young age and he was free to do so. We’ll never know what could or would have happened with a different childhood but there’s always a “if”.