A recent article in Scientific American explains the biology of why people are so willing to follow orders:

Milgram’s research tackled whether a person could be coerced into behaving heinously, but new research released Thursday offers one explanation as to why.
In particular, acting under orders caused participants to perceive a distance from outcomes that they themselves caused, said study co-author Patrick Haggard, a cognitive neuroscientist at University College London, in an email.
In other words, people actually feel disconnected from their actions when they comply with orders, even though they’re the ones committing the act.
The study, published in the journal Current Biology, described this distance as people experiencing their actions more as passive movements than fully voluntary actions when they follow orders.