When you first join murder you haha have a really bad rank (that's just how it is).
The closer your rank is to 1, the better it is.
As your rank gets closer to 1, it'll get harder to rank up (because your scores are competing with other players).
Karma is important. 1000 is the max karma and -1000 is the min. The amount of karma you have is the amount of entries that gets put into a raffle of sorts. If you win that, you're the murderer. I think it's easier to rank up when you're the murderer, so it's best if you don't shoot innocents because you'll have less of a chance to become the murderer (I've heard, but can't confirm, that if you have negative karma, you can't be the murderer. If you have negative karma you can get it back to zero by buying it in the mcpz shop.)