Herodian (late second, first half third century): Greek historian, author of a History of the Roman Empire since the Death of Marcus Aurelius in which he describes the reign of Commodus (180-192), the Year of the Five Emperors (193), the age of the Severan dynasty (211-235), and the Year of the Six Emperors (238).
The translation was made by Edward C. Echols (Herodian of Antioch's History of the Roman Empire, 1961 Berkeley and Los Angeles) and was put online for the first time by Roger Pearse (Tertullian.Org). The version offered on these pages is hyperlinked and contains notes by Jona Lendering.
Murder of Geta
[4.4.1]  But the hatred and dissension between them continued to grow. If it became necessary to appoint a governor or a magistrate, each wished to select a friend for the post. If they sat as judges, they handed down dissenting opinions, often to the ruin of those on trial; for rivalry counted more than justice to these two. Even at the shows the brothers took opposite sides.
[4.4.2] They tried every sort of intrigue; each, for example, attempted to persuade the other's cooks and cupbearers to administer some deadly poison. It was not easy for either one to succeed in these attempts, however: both were exceedingly careful and took many precautions. Finally, unable to endure the situation any longer and maddened by the desire for sole power, Caracalla decided to act and advance his cause by sword or slaughter or die in a manner befitting his birth.
[4.4.3] [19 December 211] Since his plotting was unsuccessful, he thought he must try some desperate and dangerous scheme; [so he killed his brother in the arms of their mother, and by this act really killed them both],note his mother dying of grief and his brother from treachery. Mortally wounded, Geta died, drenching his mother's breast with his blood. Having succeeded in the murder, Caracalla ran from the room and rushed throughout the palace, shouting that he had escaped grave danger and had barely managed to save his life.
[4.4.4] He ordered the soldiers who guard the imperial palace to protect him and escort him to the praetorian camp, where he could be safely guarded, saying that if he remained in the imperial palace he would be murdered. Unaware of what had happened inside, the soldiers believed him and ran with him as he dashed ahead at full speed. Consternation seized the people when they saw the emperor speeding on foot through the middle of the city in the early evening.
[4.4.5] Rushing into the camp and into the temple where the standards and decorations of the guard were worshiped, Caracalla threw himself on the ground; in the chapel, he gave thanks and offered sacrifices for his safety. When this was reported to the praetorians, some of whom were in the baths, while others were already asleep, they hurriedly assembled in amazement.
[4.4.6] When he appeared before them, Caracalla did not immediately reveal what had happened; instead, shouting that he had escaped the deadly plots of an enemy and rival, he identified his assailant as his brother. He cried out that he had with difficulty emerged victorious, after a severe struggle with his enemies; but when he and his brother had put everything at stake, Fortune had chosen him as sole emperor. His motive in thus distorting the facts was his desire to have them hear from him what had happened rather than from someone else.
[4.4.7] In gratitude for his deliverance and in return for the sole rule, he promised each soldier 2,500 denarii and increased their ration allowance by one-half. He ordered the praetorians to go immediately and take the money from the temple depositories and the treasuries. In a single day he recklessly distributed all the money which Severus had collected and hoarded from the calamities of others over a period of eighteen years.
[4.4.8] When they heard about this vast amount of money, although they were aware of what had actually occurred, the murder having been made common knowledge by fugitives from the palace, the praetorians at once proclaimed Caracalla emperor and called Geta enemy.