The murder of Britannicus, if it did indeed happen, was not plotted by Agrippina but Nero. Agrippina was ignorant of the murder and Nero alone arranged for the poison to be made by Locusta and to have it boiled until it caused instant death. Agrippina was unaware of any attempt to kill Britannicus and she is depicted in the Octavia (170-1) weeping over his body.

Nero wanted to make the murder of Britannicus public and look like a nature death in order to avert suspicion. Administering the poison was difficult since everything the prince ate or drank was sampled by his taster. To circumvent this, Nero had a harmless drink offered to Britannicus. His taster sipped it but the prince found the drink too hot and asked for some cold water to be added. The poison had been added to the water. At the first taste, Britannicus was deprived of speech and could not breathe. He collapsed and Nero casually commented that his brother had had an epileptic fit and would recover. Octavia and Agrippina showed no special concern (Ann. 13.16). According to Dio, the corpse had turned a dark due to the effects of the poison. Perhaps because of this disfigurement, a funeral was arranged in haste (Tacitus (Ann. 13.17)says it was that night, Suetonius (Nero 33.2) says the next day). A pyre had already been built in the Campus Martius and the rites were carried out during a thunderstorm. Nero, fearing his crime would be detected, had gypsum applied to the face. When the rain fell on Britannicus, the gypsum washed off revealing the crime (Dio 61.7). The next morning, Nero explained everything to Seneca and Burrus who received a special bounty paid out of the inheritance of the deceased prince. Seneca presented a version of events to the Senate claiming Britannicus had suffered a fatal epileptic fit (Ann. 13.17).