The Praetorian Guard

Assassanations
The Praetorian Guards are the worst bodyguards in history. They killed thirteen roman emperors, which they were supposed to protect. The are responsible for the deaths of Caligula, Pertinax, Maximinus Thrax, Pupienus, Balbinus, Aurelian, Caracalla, Commodus, Elagabalus, Galba, Gordian III, Numerian, Philip II, Probus, and Pupienus.

Motives
Most likely, The Praetorian Guard wanted to put those they wanted in power, instead of the emperors they had at that point.