Even European monarchs, with the Catholic church holding much of the keys to their authority and being very against it, managed to do a considerable amount of tactical relative-killing. Everywhere else it's basically the norm for monarchies that princes murder each other.

A shattering bow

A burning flame

A gaping wolf

A screeching pig

A rootless tree

A mounting sea

A flying spear

A falling wave

One night's ice

A coiled serpent

A bride's bed-talk

or a breaking sword

A bear's play

or a child of a king.

(Odin listing up some of the things a wise man never trusts, in stanza 85 and 86 of Hávamál)