General Information:

The word "her" refers to Babylon.

In her the blood of prophets and of God's holy people was found, and the blood of all who have been killed on the earth

Blood being found there means that the people there were guilty of killing people. Alternate translation: "Babylon is guilty of killing the prophets and believers and all the other people in the world who were killed" (See: rc://en/ta/man/translate/figs-activepassive and rc://en/ta/man/translate/figs-metonymy)