I dont think you can always blame an upbringing - what do we know of Hitlers formative years? anything there to suggest what he became?
I think some people possess naturally more negative personality traits than others; aggression, lack of compassion etc etc. If not addressed correctly by sensible parenting then a problem could develop.
Others behave a certain way due to experiences in life; if someone has gone through school unmercifully bullied, that could bring out undesirable behaviour later in life -depending on how they cope with that.