No matter your karma or how many innocents you've killed, you still have a chance of being murderer, however the lower your karma gets, the less likely chance you'll have to become a murderer (same with getting a gun). The highest is 1000, the middle/starting point is 0, and the worst is in the negatives.
You can still become a murderer, however it will be extremely rare because you'll most likely be playing with users who have better karma then you and therefore a better chance of being a murderer.