RobH_en_tn/2ch/20/23.md

to completely kill them and destroy them

These two phrases mean basically the same thing. The second intensifies the first. Alternate translation: "to completely annihilate them" (See: rc://en/ta/man/translate/figs-doublet)

When they had finished with

This refers to after they had destroyed them. Alternate translation: "When they had finished killing" (See: rc://en/ta/man/translate/figs-explicit)