Watch the movie “The Kite Runner” to see an excellent portrayal of this phenomenon. Great movie.
https://www.youtube.com/watch?v=muW9Ob7V7ME
Those kites look extremely amateurish. The kites we flew as kids could be maneuvered superbly, we could go left, or right or up or down at very fast speeds. The string has very fine specks of glass, and to cut other persons kite, your string has to slide several feet on his string to cut it.
If some vandal tied it across the street at a level to impact a motor cycle or bicycle rider, the string can not decapitate him. Like I explained above, the string works like a very fine saw and requires several feet of sliding on other string to cut it. Mostly like the bicycle rider will fall off the bike due to the suddenly encountered obstruction. It could may be cause a slight cut on his neck but no where near decapitation.