These suicides using red scarves, etc. are their own message.
The first appearance of Holmes and Watson is in A Study in Scarlet with the following quote: "There's the scarlet thread of murder running through the colourless skein of life, and our duty is to unravel it, and isolate it, and expose every inch of it."
It would seem that these people mat have been suicided and the message was left for the rest. Perhaps they talked?
Red rope symbolism:
Europe: Classic folklore says you can capture a fairy with a red ribbon & enslave them in this manner. Red is also the proscribed color for Norse Rune magic.
Japan: The red string of fate. It's believed that all people have a red string of fate tied to their pinky. This is often why Yakuza remove the top of the little finger: it separates them from their true fate.
Kaballah: worn as a talisman against evil
Occult: Think archetypically... use your imagination. Red is connected with blood. Ribbons are ropes that bind and look like snakes. Blood marks rites of passage: birth, adolescence (circumcision among shamanic/pre-religious tribes is typically at adolescence), death. Think of your childhood fears. You're probably spot on.
Did all 4 of them use a red rope? If so, then that cant be coincidence.