"Witchcraft celebrates / pale Hecate's offerings, and withered murder / Alarumed by his sentinel, the wolf / Whose howl's his watch, thus with his stealthy pace / With Tarquin's ravishing strides, towards his design / Moves like a ghost" (2:1)
Sextus Tarquinius, the youngest son of the King of Rome, raped his cousin's wife, Lucretia, after a dispute about her virtue. This drove her to suicide, and with that, Tarquinius had committed a double crime.
According to some, this was a pivotal event in the overthrow of the monarchy and the subsequent establishment of the Roman Republic, as this brought an end to the kingship of Tarquin the Proud.
This reference not only shows how sinful Macbeth is, but that he knows exactly what he is about to do; he is about to commit a serious crime, and more importantly, would be aware of the consequences of killing a king.
Sextus fled to Gabii, hoping to become King there, but he was killed for his previous crimes, a fate which also befalls Macbeth.