What is the deal with the assassins/bodyguards in "The Reichenbach Fall"?

Question

In "The Reichenbach Fall", we learn that several hired assassins have moved into the apartments surrounding the one John and Sherlock share. They actually save Sherlock from getting hit by cars a few times, and then when Sherlock shakes their hands, they are instantly shot by... other snipers in the area.

What?

I was convinced that when Sherlock shook Moriarty's hand at the end, Moriarty would be shot in a similar manner - that Sherlock's handshake was the signal to these invisible snipers that the recipient should be shot.

But that didn't happen. Moriarty shot himself. (So the only way my theory could have been correct is if Moriarty had been the sniper the other two times Sherlock shook hands with people in the episode. Which is impossible, since one occurred right after Moriarty drove away in a cab.)

Basically, I am confused about the purpose of these characters. Were these assassins hired to protect Sherlock or to kill him? Were they the same ones who were hired to take out John, Mrs. Hudson, and Lestrade? If so, why did they need to be killed just for shaking hands with Sherlock?

Answer 1

Firstly, the entire assassin sub-plot was an elaborate ruse to fool Sherlock into thinking he had the key Moriarty used in the break-ins at the start of the episode. So Moriarty set the whole thing up just to make Sherlock believe there was a reason he was being protected. (That the key itself didn't exist is covered in another question)

What Moriarty setup was several world-class assassins were hired to live near Sherlock and Watson. Then they were to try and get the key from him. However, they must have either been alerted to the fact that other assassins were also on the case, or they noticed each other.

This setup an armistice of sorts. Everyone kept their eyes on Sherlock and Watson, and the other assassins. Each assassin knew that to approach Sherlock and Watson meant instant death from one of the other assassins. This was because each assassin had to prevent the others from obtaining the key. Whoever has the key wins.

Moriarty lied to all of the assassins to set this up, which in the end caused Sherlock to believe himself to be more valuable than he was.

Answer 2

There is no key, after carving IOU into the apple in Sherlock's apartment Moriarty tapped a sequence out onto his knee with his fingers. It translates as:"There is no key"