The “sins” referred to here are not Christ’s sins, as Peter explicitly says in the next words: “the righteous for the unrighteous.” The point is that Jesus did not at all deserve to suffer (Matthew 27:19; John 19:4, John 19:6) while sinners—we ourselves—certainly do. That point is important in the flow of this letter because Peter had earlier instructed his readers to “do good” (1 Peter 2:12, 1 Peter 2:15, 1 Peter 2:20; 1 Peter 3:6) even if it leads to abuse, as clearly happened to Jesus.
18 For Christ also suffered once for sins, the righteous for the unrighteous, that he might bring us to God, being put to death in the flesh but made alive in the spirit,