This rhetorical question means that he should have killed him. This question can be written as a statement. The phrase "strike down" means to kill. Alternate translation: "You should have struck him down to the ground!" or "You should have killed him immediately!" (See: [[rc://en/ta/man/translate/figs-rquestion]] and [[rc://en/ta/man/translate/figs-euphemism]])
This can be written in modern units. Alternate translation: "ten silver coins" or "110 grams of silver" (See: [[rc://en/ta/man/translate/translate-bmoney]])