Tyrannicide literally is the killing of a tyrant, and usually is taken to mean the killing of a tyrant by a private person for the common good. There are two classes of tyrants whose circumstances are widely apart — tyrants by usurpation and tyrants by oppression. A tyrant by usurpation (tyrannus in titula) is one who unjustly displaces or attempts to displace the legitimate supreme ruler, and he can be considered in the act of usurpation or in subsequent peaceful possession of the supreme power. A tyrant by oppression (tyrannus in regimine) is a supreme ruler who uses his power arbitrarily and oppressively.