it was just as it is written

Here Paul refers to a scripture where Christ (the Messiah) speaks to God. You can translate this in an active form. Alternate translation: "the Messiah said to God in the scriptures" (See: rc://en/ta/man/jit/figs-explicit and rc://en/ta/man/jit/figs-activepassive)

The insults of those who insulted you fell on me

Those who blasphemed God insulted Christ. This is an indirect way of saying that those who slandered Christ were blaspheming God.