Thank you, all three of you for thoughtful and helpful answers. I should have given more detail in my original question.
This character is no rescuer of maidens. He is a villain - possibly psychopathic - and an assassin. His most notable victim was the last fully Saxon king of England, Edmund 'Ironside'.
Historically, Edmund's death is a mystery, but there have been suggestions that he was murdered in a privy, perhaps by an arrow. The "French chronicler Gaimar, reports that the murder was carried out by the diabolical contrivance of a ‘spring-bow’ – a deadly sort of booby trap consisting of a loaded crossbow that could be triggered by pressure and was known to the French as li ars qui ne fault, or ‘the bow that does not fail.’ According to this account (it is a late one, dating to about 1137), Edmund was shown into a privy rigged with ‘a drawn bow with the string attached to the seat, so that when the king sat on it the arrow was released and entered his fundament.’"
I do not have to go with Gaimar's account, but I quite like it. I also like the particularity of the fellow being a military crossbowman in a non-crossbow society. This seems more interesting than making him a Welsh longbowman - another possible approach. So, Geezer, it looks like you've pretty much hit the nail on the head. And your technical details are very interesting. You've given me some good pointers for further research - I know nothing of crossbows - and I hope you will permit me to throw you some more detailed questions, if necessary, at a later date.
Again, big thanks to all of you. And the topic remains open for any further ideas that may spring to mind.