When it comes to natural law (in Catholicism perspective), I understand that there is wrong to kill, also when it's suicide, but is it wrong with self defense? And what if there is an accident? Like you forget to turn off the oven at home and someone on the block gets hurt, is that the same as taking a life? On purpose?
Generally speaking the intentions/motivations of the person make it right/wrong or allowable/forbidden.
Sin has always been rebellion against God—generally speaking, doing something that you believe to be wrong. The Jews had some mandates about accidental sins, but with the atonement that Jesus made, I don't see how there's a need for any form of repentance for things that you did not knowingly choose to do (or not do, in the case of something you ought to have done).
If someone dies due to my negligence or irresponsibility, then that might constitute a sin on my part, because I made that choice. Accidents that you could not have controlled do not constitute a choice. You're choices are what will condemn or exonerate you.
2 question are asked here and 2 assumptions are made.
Assumption 1) Killing is wrong. If this were true in all cases, then accidents, suicides, self defense, killing during armed combat, abortion and other forms of premeditated murder would have the same gravity.
Assumption 2) Suicide is wrong. This is true "suicide is always as morally objectionable as murder" pope John Paul II (Evangelium Vitae). Even assisted suicide and euthanasia, but one needs to make a distinction in cases of euthanasia between withholding extraordinary means of keeping a person going and withholding the normal means (food and water).
Question 1.) Killing in self defense is not wrong, it's not the same as murder. Reason dictates that man doesn't have to die just because someone else desires to kill him. Self preservation is in the Natural Law, not against it. Pope John Paul II applies to reason when he says the death penalty should never be used "except in cases of extreme necessity". One of those reasons is that society is unable to protect itself.
Question 2.) Accidents, a person is not morally culpable for something which they did not intend if they followed their consciences. A person should seek to know the truth and not act with any doubt in their conscience. If you forgot you left the oven on and had an inkling that you did that, then you owe it to your neighbors to check. It is unlikely that any true accident could have the gravity of premeditated murder, but it could come close!
Biblically, there is no exception. By both word and deed, Christ taught that it is wrong to take a human life or hurt another person, even in self-defense. "But I tell you, do not resist an evil person. If anyone slaps you on the right cheek, turn to them the other cheek also." Matthew 5:39 (NIV).
But actions speak louder than words, and Christ did not resist his arrest and execution. When Peter picked up a sword and attempted to defend him (cutting off the ear of the high priest's servant/slave) Jesus forced him to put the sword down. John 18:11-12
Catholicism and many other churches have made doctrinal exceptions, but these do not have any Biblical basis that I can find.
Even under the law of Moses, causing an accidental death was not treated as murder. "If anyone kills his neighbor unintentionally without having hated him in the past— as when someone goes into the forest with his neighbor to cut wood, and his hand swings the axe to cut down a tree, and the head slips from the handle and strikes his neighbor so that he dies—he may flee to one of these cities and live, . ." Deut. 19:4-13