...were adopted as babies and raised by the world's best parents...
Do you think they would have turned out differently / better?
Yes
Or are some people born with a genetic defect that predestines them to become evil?
No
Explaination: Genetics do change a lot of things, but not that kind of stuff.
Let's ignore handicaps for the sake of this post, Hitler didn't have those, or wasn't born with those, I've never heard of Manson, but it's probably the same with him.
What do genetics change in the minds of people? They change lots of small things, like strength of certain emotions, impulsivity in you, sensitivity to outside impulses, ability to learn faster/slower, and so on.
The rest, such as hatred/cruelty, selfishness, pride, but also love and affection, goodwill and goodness are all learned/influenced in you AFTER birth.
Also, a lot of "mental disorders" are also "learned" (not the correct word) after birth. Usually in the first few years of life. Such as phobias, hyperactivity, social anxiety, and so on.
Say someone is born as a person with instable emotions.
You get very loving parents, not much negativity around you, a calm childhood, and so on. (I'm not talking about overprotection).
That person will be instable, and maybe easly influenced, but has learned to love, has a strong morality, and so on in the earliest years, so it will turn out to be a very loving and kindhearted person.
Say that person has abusive parents, maybe a rapist dad, and so on, and sees a lot of violence and abuse, maybe death in early years. And say that person was also born with a huge intelligence/logic, and as a very tall strong person.
That person will have a weaker or nonexistent conscience/morality, will be used to being taller and stronger, will see patterns/solutions in everything, and so on.
That person will have a chance to be a serial killer, not always, but way way more likely then the average person.
These are of course two extremes. Lots of inbetweens exist. That's why while most people care and love for others, they are also selfish and lack some compassion. All in different degress. All "inbetweens" (the grow-up situation).
Hitler was born as an impulsive and emotive person. I don't know about his childhood, but he had bad experiences as a young adult, he also had a lack of compassion, and grew a hatred. Then he had bad influences (war, nationalism and propaganda in WW1), and then he got power. Of course he was the tyrant he was!
It is still born the same person, but the surroundings/influences after birth have either turned the person loving and kindhearted, or evil and without mercy.
Young kids become and immitate what they experience and see the most around them. (Not talking about everything, there's exceptions and "greys" for a lot)
So say Hitler was born in a very warm and loving family, and wasn't subjected to big violence and bad habits/influences, he would also most likely be a very loving and warm man.
chance and Nocturne were very correct in this topic, Law and K12 weren't so correct.
Off-topic but kinda on-topic:
One exception of a mental disorder might be the food or stuff the mother consumes while being pregenant. Say a mom consumes lots of sugar and fatty foods, the fetus will get an addiction to fatty foods and sugar, and will get more energy. That child might be born with ADHD of some sort. But this certain situation, I also see as "after birth", even though it is technically before birth. Because it has more to do with influences, then with inner nature "born" in a person.