The incident happened at Jhamela village under Tiwari police station in the district here, police said.
The accused identified as Sanju, 30, has been arrested, they said.
The victim, Bittu, was allegedly having an illicit affair with Sanju's wife.
Today Bittu was seen in an "objectionable position" with his younger brother's wife following which Sanju beat his elder brother to death, police said.
A case has been registered and the matter is being investigated, police said.