Thanks Moriarty :D

Well, technically he was the one "sponsoring" the murders, but he wasn't the one who actually killed her.

^precisely, Moriarty was responsible. The cab driver was just a pawn and his name wasn't mentioned.

oh yeah he was the guy paying the old man sorry forgot been a while

GET THE HELL OUT OF HERE IF YOU DON'T WANT SHERLOCK SPOILERS!

Moriarty would pay this man every time he murders someone. The reason he agreed to murder people was because he had a terminal disease, could possibly die at any moment, and needed a way to fund his kids. So instead of bloodily killing people, the cab driver would take people who need a ride, and then take them to a secluded area. He would then force them to take a pill. The catch was, that there were two bottles so completely identical pills, and whichever pill the person takes, he would take the other. While the victims think it's a game purely consisting of chance, the cab driver claims that he understands the psychology of people and their emotions, and because he's such a genius, it causes him to win all four times, inevitably killing four people. The smart thing was instead of having evidence everywhere, he'd have them swallow pills and would leave them be, to make it look as if it was their own choice (basically suicide).

Moriarty initiated the cab driver, therefore sponsoring him. Otherwise the cab driver would've found another way to support his kids, and it probably wouldn't involve murder. Anyways since I've watched is episode like six times, and I thought it was really clever, that's what they meant when they said that Moriarty was responsible. Unless you're asking for me to explain why Umbridge deserves death more than Voldemort, to which the answer is that Umbridge was an asshole. Voldemort was a sociopath that just wanted to live forever.

Thank you so much :D

That's actually an excellent point :| Sherlock never wore scrubs