Decius is a brutal, amoral and an exceptionally vicious Roman general. He's a secondary villain in Shadow of Rome. He is the true murderer of Caesar. He will kill anyone who knows it. He murders Agrippa's mother and enjoys killing innocent people. He is the leader of the Valcross stable gladiators. When Agrippa kills one of his Valcross troupe, Decius takes it as a personal insult, as he could not return to Rome with humiliation inflicted by a low-life gladiator. So he faced Agrippa against his entire Valcross stable, in a series of matches, only to see them all defeated. In the end he battles Agrippa by himself in melee only to underline his apparent narcissism and the fact that he killed Caesar. Decius' last words were an exclamation of his self-perceived overblown military genius. Decius is killed by a low-life gladiator and died as a murder and a disgrace general. To him getting killed by a gladiator is a dishonorbale death and he'll never be recognize as a general.
Agrippa kills him because Decius was a murder and he murdered his mother.