Excerpts from Books and Wikipedia
"[Pompey] had by the late summer of 48 B.C.E. become a desperate fugitive. Defeated at Pharsalus in northern Greece by his rival Julius Caesar, he had fled by sea to Egypt. Here, Pompey sent a message to the country's young king, Ptolemy XII, requesting refuge. . . . They reasoned that Caesar would be enraged if they gave Pompey shelter, but also argued that Pompey could be a future threat if they turned him away. The safest solution, therefore, was to kill him." [2001 Days]