Would you say that 'in cold blood' itself constitutes a metaphor?
I ask because of the quote taken from the book of that title by Truman Capote that I use as a signature for this blog.
The metaphor being that heat alludes to one of the four humours, i.e. choleric, or hot; the one of the four that equates to violence, vengefulness, or bad-temper, so an act committed in cold blood insinuates the opposite, i. e. pre-meditated, calm, and unemotional. Thus 'in cold blood' is a not a direct description but a metaphor for an attitude or state of mind.
If this is so, then those searching for a metaphor of 'in cold blood', as people do, are wasting their time for they are looking for a metaphor of a metaphor.
One could, I suppose, arrive at a Proustian laden metaphor upon a metaphor:
'He struck her down in cold blood that was chill as an icicle broken from the roof of the only chalet located at 5,000m on Mount Everest, stored in a deep freeze, and flown the 6,827 miles home to Milton Keynes to be used to make the perfect dry martini. He was that cool. She, on the other hand, as her life blood drained away, was merely cooling."
(Though this, of course, is a simile.)