“Death nail” is a result of confusing two expressions with similar meanings.
The first is “death knell.” When a large bell (like a church bell) rings—or tolls—it knells. When a bell is rung slowly to mark the death of someone, it is said to sound the death knell. But “death knell” is more often used figuratively, as in “his arrest for embezzlement sounded the death knell for Rob’s campaign to be state treasurer.
Another way to describe the final blow that finishes someone or something off is “put the last nail in the coffin,” as in “a huge budget cut put the last nail in the coffin of the city’s plan to erect a statue of the mayor’s dog.” Something not yet fatal but seriously damaging can be said to “drive another nail” in its coffin.