A.T. Robertson Commentary Acts 12:2

"And he killed James the brother of John with the sword." — Acts 12:2 (ASV)

James the brother of John (Ιακωβον τον αδελφον Ιωανου). He had been called by Jesus a son of thunder along with his brother John. Jesus had predicted a bloody death for both of them (Mr 10:38ff.; Matthew 20:23). James is the first of the apostles to die and John probably the last. He is not James the Lord's brother (Galatians 1:19). We do not know why Luke tells so little about the death of James and so much about the death of Stephen nor do we know why Herod selected him as a victim. Eusebius (H.E. ii. 9) quotes Clement of Alexandria as saying that a Jew made accusations against James and was converted and beheaded at the same time with him.

Killed with the sword (ανειλεν μαχαιρη). The verb is a favourite one with Luke (Acts 2:33; 5:33,36; Acts 7:28; Acts 9:23–29; 10:39, etc.). Instrumental case and Ionic form of μαχαιρα. The Jews considered beheading a shameful death as in the case of the Baptist (Matthew 14:10).