I'm just wondering: why "red"?
- Anybody can ask a question
- Anybody can answer
- The best answers are voted up and rise to the top
On etymonline you will find that it is, presumably, from the blood on hands.
There are other more detailed articles around, quote:
George Macdonald Fraser, in his "Steel Bonnets", a history of the feuds and raids on the English-Scots border, says that pursuing raiders across the Border was not only allowed but in some circumstances compulsory, and
If this is right (and Fraser is one of the best sources on this subject), it's from the 16th century if not before.