I want to purge the world

I have for the longest time wanted to kill several people, not even people I know, I've just wanted to kill people, several people, dozens of people. Its not because I'm mad at them or anything, I just feel that some people around this world need to die. I keep hearing a voice in my head that says to kill random people on the street, people in school, and even form a society of assassins and kill people globally for the "greater good". The voice keeps telling me "it is in the name of god, these people are evil, I must weild the blade of righteousness and purge the world." I know that it's a lie and that all it wants is to kill people and its just using religion as an excuse. The part that got me nervous is that I dont know if the voice is just in my head or not. I dont know if I'm just going psycho for blood-lust, but the voices are driving me insane. I dont know what to do, but I hope the voice will go away before I start to listen to them. I dont want to hurt anyone.

Worst part is that I was told to kill my friends too.