It dates back to 1934 when the leadership of the Brownshirts in Nazi Germany were arrested and then murdered on Hitler's orders.
Harold Wilson used it when the Tory PM, McMillan, sacked half of his cabinet.
Hard to answer. This term is generally associated with Hitlers purge of The SA between the 30th June-2nd July 1934, it is also used to describe Mcmillans sacking 7 cabinet members in 1962 and as far back in history, though quite topical in some ways, as the murder in Kabul of Alexander Burnes the British Consul to Afghanistan in 1841, But who actually coined the phrase I'm afraid you'll have to take your pick