First, "The Top Way"...this is an incredibly insensitive title.

Second, incremental acts to building towards something bigger is a very simplistic way of looking at how people committed such crimes. Perhaps look into the dehumanising process or the banality of evil as proposed by Hannah Arendt to have a better understanding of how evil occurs.

Finally: Peer pressure and normalization also play a part rather than labelling everything in simple terms as "all great evils perpetrated on populations started out small and grew..."