After spying this delicious and lonely nodeshell, I decided to think upon exactly how this would work. Taking the remains of a recently snapped string, (yes, it actually was my g-string. Shut up.) I embarked on a mystical journey to find out how, well, to kill someone with a guitar string.
First and most obvious method- strangulation. I'm sure you're all aware of the piano wire/fishing line around the neck garotte technique, seen utilised by spies and Mafia goons in many a film.
Due to a surprising lack of willing participants, I decided to see the effectiveness for myself. Looping the copper-wound string around my neck, it's pretty efficient at cutting into my arteries and windpipe with a small amount of pressure. In a handy addition to your standard wire, a guitar string comes with a little metal loop at one end of the string. It looks like this, for those that've never seen one.
Of course, you can then make a handy minature noose by feeding one end of the string through the ring, creating yourself a very nasty wire loop. Makes strangling that much easier.
The only other imaginable use for a guitar string is also very effective- wielding it like a whip. After practising for a little while, and making an extremely painful trial against my own leg, it is confirmed that a guitar string leaves very, very painful welts. It probably won't kill them, but hey, it hurts a whole lot.
After making various attempts to find other nefarious things to do with a guitar string, well, there wasn't much success. The thin end of the string can be quite pointy, but any attempts at stabbing are frankly pathetic. As a comparison, imagine trying to stab someone with a very bendy sewing needle. Yes, it is that useless.
After examination of various string types, I have come to the conclusion that a wound 3rd (g) string from an acoustic guitar set is the most efficient string for dealing various forms of bodily harm. A plain string's too light to be effective at the whipping side of things, while the heavier strings (e and a, etc) are just kind of clumsy. But the g string... yes. It just works.
Stop snickering, goddammit!