The Greek word used here is the word axe
used as verb. The result of the work of the axe is that the head has been severed from the body. Severing head from body is an obvious method of showing to onlookers your victory over your opponent (1 Samuel 17:51; 1 Samuel 31:9). So a severed head symbolizes triumph for the victor and defeat for the victim. In the present context we are not to think that only those martyrs who were beheaded for their faith need and receive justice, but all martyrs do. Rather, the reference to beheading symbolizes the apparent victory of Satan over these martyrs.
There is a delightful irony here: while martyrs have lost their heads, the Lord has captured the head of all evil. The latter is chained in the abyss; the former are in the presence of the King of kings as coregents with Christ. Here is a clear illustration of Jesus’ words from Matthew 19:30.
4 Then I saw thrones, and seated on them were those to whom the authority to judge was committed. Also I saw the souls of those who had been beheaded for the testimony of Jesus and for the word of God, and those who had not worshiped the beast or its image and had not received its mark on their foreheads or their hands. They came to life and reigned with Christ for a thousand years.