If someone commits fratricide, they kill their brother.
[formal]
fratricide in British English
(ˈfrætrɪˌsaɪd, ˈfreɪ-)
noun
1.
the act of killing one's brother
2.
a person who kills his or her brother
3. military
the destruction of or interference with a nuclear missile before it can strike its target caused by the earlier explosion of a warhead at a nearby target
Derived forms
fratricidal (ˌfratriˈcidal)
adjective
Word origin
C15: from Latin frātricīda; see frater1, -cide
fratricide in American English
(ˈfrætrəˌsaɪd)
noun
1.
a.
the act of killing one's own brother or sister
b.
the act of killing relatives or fellow-countrymen, as in a civil war
2.
a person who kills his or her own brother or sister
Derived forms
fratricidal (ˌfratriˈcidal) (ˈfrætrəˈsaɪdəl)
adjective
Word origin
Fr < LL(Ec) fratricidium < L fratricida, one who kills a brother < frater, brother + caedere, to kill (see -cide); (sense 2) ME < OFr < L fratricida