Nero claimed that Britannicus died from an epileptic seizure , but ancient historians all claim Britannicus ' death came from Nero 's poisoning him . Supposedly , he enlisted the services of Locusta , a woman who specialized in the manufacture of poisons . She devised a mixture to kill Britannicus , but after testing it unsuccessfully on a slave , Nero angrily threatened to have her put to death if she did not come up with something usable . Locusta then devised a new concoction that she promised would " kill swifter than a viper . "