Romans 5:8

The third term used to describe those for whom Christ died is "sinners" ("wicked"; cf. Romans 3:23), neither righteous nor good. Paul here was contrasting the worth of the life laid down, Jesus Christ’s, and the unworthiness of those who benefit from His sacrifice. Whereas people may look at one another as meriting love because they are righteous or good, God views them as sinners. Nevertheless God loves them. His provision of His own Son as our Savior demonstrated the depth of His love (John 3:16).

The preposition in the clause "Christ died for (huper) us" stresses the substitute character of His sacrifice. It also highlights the fact that God in His love for us provided that sacrifice for our welfare.