KERINGET, Kenya — At first the violence seemed as spontaneous as it was shocking, with machete-wielding mobs hacking people to death and burning women and children alive in a country that was celebrated as one of Africa’s most stable. But a closer look at what has unfolded in the past three weeks, since a deeply flawed election plunged Kenya into chaos, shows that some of the bloodletting that has left more than 650 people dead may have been premeditated and organized. Leaflets calling for ethnic killings mysteriously appeared before the voting. Politicians with both the government and opposition parties gave...