The book by Charles R. Browning discusses this. Ordinary Men is the true story of Reserve Police Battalion 101 of the German Order Police, which was responsible for mass shootings as well as round-ups of Jewish people for deportation to Nazi death camps in Poland in 1942. Browning argues that most of the men of RPB 101 were not fanatical Nazis but, rather, ordinary middle-aged, working-class men who committed these atrocities out of a mixture of motives, including the group dynamics of conformity, deference to authority, role adaptation, and the altering of moral norms to justify their actions. Very quickly three groups emerged within the battalion: a core of eager killers, a plurality who carried out their duties reliably but without initiative, and a small minority who evaded participation in the acts of killing without diminishing the murderous efficiency of the battalion whatsoever.
While this book discusses a specific Reserve Unit during WWII, the general argument Browning makes is that most people succumb to the pressures of a group setting and commit actions they would never do of their own volition.
Ordinary Men is a powerful, chilling, and important work with themes and arguments that continue to resonate today.