When raised to believe you're superior to others you start to see yourself above human beings. Easier to believe that station and power make you a supreme existence, since you can get away with doing things like this.
Thus the 'evil noble trope' is actually fairly realistic and more likely to happen than to not, since having that kind of upbringing while remaining a bastion of morality and kindness is an extreme effort. ... Doubly so because nice guys finish last in power struggles, only the dirty ones get ahead unless we're talking about world-class luck, skill, natural talent, and hard work making it almost an even race for the 'good noble'.