As it is written - Psalms 44:22. This passage the apostle quotes not as having originally reference to Christians, but as “aptly descriptive” of their condition. The condition of saints in the time of the psalmist was similar to that of Christians in the time of Paul. The same language would express both.
For thy sake - In thy cause; or on account of attachment to time.
We are killed - We are subject to, or exposed to death. We endure sufferings equivalent to dying; compare 1 Corinthians 4:9, “God hath set forth us the apostles last, “as it were appointed to death.”
All the day long - Continually; constantly. There is no intermission to our danger, and to our exposure to death.
We are accounted - We are reckoned; we are regarded, or dealt with. That is, our enemies judge that we ought to die, and deem us the appropriate subjects of slaughter, with as little concern or remorse as the lives of sheep are taken.