As time machines become more common, the Time Police find themselves working harder and harder. Their job is to maintain the internal consistency of history, and to halt or amend paradoxes caused by overzealous time-travelers. When chronological disaster is detected on the mightly Hourglass 3000's quantum circuitry, the Time Police swing into action. Using subterfuge, ingenuity, and highly advanced technology, they redirect the river of causality back into its natural path. And then, for what they claim is poetic justice, they kill the time traveler's grandfather.
Oh, not before he or she is born. No, the Time Police would never invite paradox that way. Instead, they travel back to the meddler's childhood, to a time when youth and grandfather are sharing a private moment together. They shoot the elder, and to the time traveler-to-be they utter four words, which he or she will never forget: "This is your fault."
Research is as of yet unclear on the number of time travelers inspired by feelings of childhood guilt and filial duty.