Of course you’re right that trapdoors aren’t only used for hanging, and I discuss that at the end of the article:
“So how will people — from Hillary Clinton to the guardians of political correctness — react to Bob Herbert’s line? Are hanging metaphors suddenly acceptable when used by pundits of the left? Will Herbert claim he used the trap door image only to suggest that Hillary wants to make Obama suddenly disappear? What if a conservative pundit had said he’d like to open a trap door beneath Obama? Odds his explanation along such lines would be accepted on the left?”