Negligence is defined as doing something a reasonable prudent person would do under the same or similar circumstances or not doing what a reasonable prudent person would not do under the same or similar circumstances. Note that the definition is not a strict liability concept, that is one in which liability attaches solely because the event occurred without consideration of culpability or lack thereof.
So in the case of unintended discharges, the question becomes "do reasonable prudent people act in the manner the defendant acted on the occasion in question?" Sometimes the answer is "no" and sometimes the answer is "yes." It could not be reasonably argued that a "reasonable prudent person" would ever point a firearm at someone unless deadly force or the threat of deadly force would be justified. Terrifying your buddy by pointing a gun at him as a joke is negligence.
Remember, the focus is on the act or omission, not the subjective intent. For example, my hunting buddy never had an intent to walk around with his finger on the trigger of his shotgun while we were dove hunting. In fact he never had an intent to put his finger on the trigger at all unless he was tracking a bird with the intent to fire. Nevertheless, it was a hot afternoon and his sweaty finger was properly indexed on the side of the trigger guard when he stumbled on something in the field and fell. His sweaty finger slipped off the trigger guard, hit the trigger and the shotgun went off. No one was hurt and the only injury was to his pride, but that was a matter of luck. The gun could have been pointed at any of us in the hunting party. He was not negligent, although two well-accepted firearm safety rules are 1) never point the muzzle at anything you are not willing to destroy; and 2) keep your finger off the trigger until you are ready to shoot. At least the second rule was violated, yet it was an accident not negligence. Note also that it was an accident even though he could have done things differently to prevent the unintended discharge, such as keeping his trigger finger on the stock while walking.
Here's another example. Do "reasonable prudent people" who are hunting or defending themselves ever shoot blindly at movement or noise? No, of course not! "Reasonable prudent people" identify their targets before firing. Are "reasonable prudent people" ever mistaken about the identity of a target while hunting or engaging in self-defense? Yes, sometimes they are mistaken in spite of making the best effort they can under the exiting circumstances. Whether this mistake in perception rises to the level of negligence will depend upon the surrounding circumstances, but, again, this is not a strict liability situation that dictates "it happened, therefore it was negligent."
There are countless examples that I can list, but I think these two exemplify my point. Mere occurrence does not equate to negligence. Again, everyone can set their personal standard to a strict liability level, but I choose to stick with Texas law.
Chas.