What kind of people are willing to undertake such dreadful business as murdering innocent women and children?

People who want power without being responsible with it. After gaining power there is always something else that is wanted. The quote “Great power comes with great responsibility” fits with the plot since Macbeth will do anything to eliminate people in his way in order to be the only one to have this power

What kind of society produces such callous killers?

A place where society is put into a system like the feudal system where royalty is at the top and peasants at the bottom. Royalty is treated with so much respect, while peasants have to do chores and live in poor conditions. Killers will often people who are not treated the way they think they should be treated and this is what triggers them to do big decisions such as murder.