Tupac Shakur's made the phrase from "kill" and "illuminati." He believed that the Illuminati were plotting a new world order and wanted to stop them. He claimed that he had discovered this after reading Machiavelli's "The Prince."
Unfortunately, Tupac was clearly lying, because the Illuminati was a group created in the late 1700's, while The Prince was written in 1513. See a problem there?
Furthermore, The Prince was written while Machiavelli was imprisoned as a political prisoner. Most of the book, when looked at in context, was meant to be sarcastic about the upper classes (not some serious guide on how to rule effectively)
In summary, anyone who believes in the Illuminati is highly disillusioned to think of Tupac as some sort of prophet regarding their existence.
Tupac claimed to be inspired to create the word "killuminati" while reading "The Prince," we now know this to be false.