People usually don't run around with a loaded gun in their hand. So this isn't an accident. If you have a gun, you are normally required to keep it separate from the ammunition. If you load it, you have to put the safety on. If you don't put the safety on, you have to handle it very carefully, not runnig round and bumping into people, but having it firmly under your control and not pointing it into the direction of people. This is ridiculously far away from negligence or even gross negligence.
In Germany, it would be murder. You created intentionally a situation where you knew or should have known that people could get hurt or even killed, and you didn't care. You don't have to intend to kill someone, it's enough to knowingly create a dangerous situation and not care that someone could be killed.
In the USA, in many states the fact that someone died as a result of a crime you committed will count as murder. That has included cases where two people committed a (serious) crime and one was killed by a security guard - the other went to court for murder.