In Psalms, the words concerning the psalmists' hating of their enemies (18:37-42; 40:14-15; cf. Prov. 25:21-22), their asking of God to avenge them (3:7; 43:1; 54:1-3), and their cursing of others (35:4-7; 70:2-3; 109:1-15) are surely not God's words, but words that come out of the psalmists' mouth from their natural sentiment while they are praising God. Yet they are recorded by the Spirit of God in the Holy Scripture to serve God's purpose to expose the degree of the psalmists' spirituality that, on the one hand, they love God and seek God, but on the other hand, they are still so natural in their sentiment. While the psalmists were praising God, they were also asking God to avenge them. This is recorded in the Bible in order to let us know that regardless of how much we praise God, we can still be natural.
Peter's word in Matthew 16:22, "God be merciful to You, Lord! This shall by no means happen to You!" is clearly not the word spoken by God but the word spoken by Peter, occupied by, usurped by, and even having become Satan, as the Lord rebuked him, saying, "Get behind Me, Satan! You are a stumbling block to Me, for you are not setting your mind on the things of God, but on the things of men" (v. 23).
It was after Peter received the Father's revelation concerning Christ's person and the teaching of Christ concerning His building of the church and heard the Lord tell him and the other disciples that He must be crucified and raised (v. 21) that he spoke this satanic word. In Matthew 16 Peter received the clear revelation from the Father that Jesus is God's Christ and God's Son. Then the Lord told him that He was going to be crucified and resurrected on the third day. But Peter still spoke a word to the Lord which caused the Lord to call him "Satan." Peter received the divine revelation from the Father and shortly after this he spoke the word of Satan. This shows that we can receive a genuine revelation from God and afterward speak Satan's word. This is recorded in the Bible in order to teach us that we have to be watchful and pray that we would not be deceived.
Peter's intention in speaking this word was to frustrate the Lord from going to accomplish His death and resurrection for the accomplishment of God's eternal redemption and His dynamic salvation. Peter was usurped and possessed by Satan to frustrate the Lord from going on to accomplish God's redemption.