Tupac was a religious fellow, and believed stongly in the experience of enlightenment. After reading The Prince by Machiavelli while in prison he found out about the enlightenment era society, Illuminati, a group centered around knowledge and enlightenment, that deviated from the theocratic and biased views of the church and government, known for many controversies and conspiracy theories.
Tupac had combined kill with illuminati to refer to gangs, fraternities born in the thug life, which engaged in criminal activities. Kill refferred to the violent nature of gang life, and Illuminati refferred to the enlightenment gained from experience, thus Killuminati.
It had no relation to any conspiracies, contrary to popular believe.