You're missing something here, all of this operates on a principle that says that you do not take life without a good cause, that you harm nothing, especially a person, without cause. You do not destroy without purpose, and where a person is concerned we require that purpose to be especially valid. Even when killing a non-sentient tree we expect some justification, however minimal.

Pro-gun is not pro-death, to the contrary, it simply recognizes that a person has a right to protect their own life from threat. You do not have a right to kill someone on accident, but we hold that murder can not be committed by accident.

It definitely does not mean an accidental killing is 'No big deal', you are, sorry to offend, a fool to say so. It is a very big deal, so is a hurricane that sweeps through a state killing people, but a hurricane is not a murderer and there's no means or reason to punish it. If I hit a bump on the road, swerve and hit someone I've killed them, and it is a big deal, but I am not a murderer, punishment serves no purpose and is not morally justified. Same, if I am convinced my life is in danger I must take action to protect it. After the fact other will wish to know why and to ask if my impression of peril was legitimate and reasonable, and my actions in accordance with that legitimate and reasonable. If the answer to both was yes, then the matter is concluded, it doesn't mean it wasn't a big deal. If a mugger attacks me and I flee, breaking through the door into someone's home for shelter and a phone, and in doing so expose an act of adultery that is a big deal and that person will come to harm in a divorce which was because of my actions but not because I was acting unreasonably or with intent to harm them.