They will be murdered by the Antichrist because, having accepted Christ as Savior, they will refuse to worship the Antichrist and are therefore beheaded.
These are the people God is referring to when He says:
4 And I saw thrones, and they sat upon them, and judgment was given unto them: and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands; and they lived and reigned with Christ a thousand years. (Revelation 20:4).
People will be saved during the Tribulation but those who have accepted Christ since His death on the Cross up until the minute the Rapture takes place will not have to go through those seven years.