Rwandan genocide
The Rwandan Genocide was committed in a very organized way.  For example, the genocide's leaders made lists of people who were against the Hutu-led government.  They gave these lists to militias, who went and killed those people, along with their families.  The militias also blocked roads so they could check the identification cards of everyone who came by.  These cards had a person's ethnic group listed on them.  When they found a Tutsi, the militias would kill the person.  Often, they used machetes.  When Tutsis tried to hide from the killers, the militias would search every building in an area, inch by inch, until they found the people who were hiding.  Buses even drove the killers from massacre to massacre.