Execution cannot be pardoned. Where would you put a comma and why?

The correct placement for a comma in this sentence would be after the word "pardoned." This is because the phrase "cannot be pardoned" is an independent clause, and as such, it needs to be separated from the rest of the sentence with a comma. This is known as a comma splice, which is used to separate two independent clauses that are closely related in meaning. By adding the comma, the sentence becomes more clear and easier to read.