"who am I to decide who should live and die?"
As you have probably heard, the conventional answer to that question is that when shooting for self-defense you are not trying to kill. What you are entitled to do is stop the life threatening behaviour, etc... Shooting as a means of doing that is never legitimately a mater of deciding whether the aggressor deserves to die. Of course, if shot, the aggressor may die, but that is beside the point. The victim's responsibility is to assess the personal danger they are in, and to choose the least escalating way of stopping the attack.
A firearm that shot a weak cartridge with poisoned bullets, the kind of thing that would not be able to stop an attack, but would very likely kill the aggressor at a latter time, that might well not be a legal response to the attack. In some jurisdictions, knives are treated that way. It is perceived that they are extremely lethal but not good for self defense. Whether that is correct or not, one can see the wheels tuning. Weak pistols may fall into the same category though I doubt anyone has been charged on that basis
Typically the victim is the expert because the law may be written so that the key issues are whether the victim perceived themselves to be in danger, and used the most sensible non-escalating means to stop the attack. What is called for is the victim's judgment relative to the situation, not the individual. Whether or not cliffs are a well meaning natural feature or not, could probably be argued at length by philosophers. Whether or not one has currently fallen from one is well within individual expertise.
Handguns are pretty reasonable tools for killing/hunting either bears or moose. For larger bears, or moose, a handgun is not powerful enough to be a stopping gun. It's more than worth the try, but if you are attacked, handguns are marginal in effect compared to rifles for stopping large and motivated animals.