Hit The Nail On The Coffin

A Malaphor, or an idiom blend, combining "hit the nail on the head" (to do something exactly right) and "another nail in the coffin"(something that will harm or destroy someone – e.g., cigarette) to mean someone succeeded at doing something that may lead to their demise or the demise of another or just that they did a great job at something that they probably shouldn't have done at all.

Best used sarcastically.

"You did a great job of pissing off your ex. Really hit the nail on the coffin back there."
"What?"