This is a horror/thriller with supernatural undertones about two former soldiers who now work as hitmen, having left the army after an unspecified event going wrong in eastern Europe. They are hired by some wealthy, shadowy affulent types who offer them top dollar to kill the names on a list given to them. Take a guess where the title comes from. There are hints dotted through the film that the plot is tied in with Arthurian legend, some of which are very casual and a second viewing helps pick up on them.
Michael Smiley (has possibly the best surname ever*) and Neil Maskell play the hitmen and are both flippin' excellent, as is MyAnna Buring who plays Maskell's wife and looks stunningly beautiful in every shot. Which is a bit spazzy to point out, but she probably flushes away better genes every morning than what composites most of the mutants I encounter day-to-day.
The end has divided opinion greatly, I think it works but you'll have to decide for yourself. Well, I'm not your nan, make up your own mind.
Although Nan Plop would be a good name for a blog.
Anyway, watch it, it's great.
*Actually, this guy has the best surname ever: http://www.imdb.com/name/nm0125405/