I did not ask if missionaries were ever killed: I asked about crucifixion.
The Wikipedia article crucifixion is permeated with question marks (somewhat self-doubting as it were), but while it covers Rome, Christianity, Islam, Japan, China, Ukraine, it has no reference to Africa. In popular fiction (and non-fiction) there are scattered references to ant hills, but no references to ant-hills combined with crucifixion. Crucifixion is lengthy, resulting most usually in death by asphyxiation. It is a bit silly to place it on an ant hill. There is a sort of school-boy quality to Eliot's attempts at obscenity and/or torture.
P.S. The armies carrying on mass slaughter in Africa over the last 50 years have for the most part been armed and supplied by the U.S., U.K., or France. Eliot knew Heart of Darkness, from which he could have learned something about torture.
P.S. 2 How does one gloss the tone of
Like the river with its cargo of dead negroes, cows and chicken