This has always been an interesting question. But, to me it is a easy question to answer. The typical problem that comes along with answering this question is that people are trying to answer it by distinguishing between a right or wrong action. Any person could see their actions as right by them even if another person thought them as immoral. This is because a person's immoral action could serve as purposeful to the person acting it out. For instance if someone killed for the purpose of self defense, one could see this situation as being immoral for killing, yet justified or a right action for the purpose of survival. This subjective situation is where morality seems definable by the moment. Yet morality should be universal because emotions are universal. All humans can feel similar emotional reactions to completely different levels of circumstances. For example a starving child in Africa can feel the same level of joy when finding bread, while an American business man feels joy off of finally making that big sale. Both of these causes of joy would be taken for granted if presented to the other person. So, the emotion is subjective to the individual, yet it is still a universal experience that can define moral behavior. Although it is true that emotions are subjective, there is still this knowledge that a positive emotion represents goodness. And although one could argue that a killer could feel positive emotion when murdering... the knowledge that this is immoral is still known by the wise. For a wise person takes everything into account before concluding, just as a person writing a scholarly essay has to be unbiased. So, if every person's knowledge on what constitutes right action was taken into account, then the the right answer would arise because of a person's understanding of what actions constitute positivity. Now you could say that a killer would skew that answer... but then you have to ask... did they take everyone's opinions into account? Do they fall within the definition of wise? In which case a killer's immoral nature is not ignored by the wise yet it does not subvert the truth and meaning of virtuous behavior.

So all in all... the person that killed for self defense would not have had been confronted with the need to kill had the original attacker taken their victim's perception into account and/or had been wise enough to not want to harm in the first place. To focus on if it is immoral to kill in self defense is just diverging from the truth that immorality has negative causes...

I feel that this answer also brings light to the topic of what constitutes harming certain animals verses others (including humans)... in that you could ask... would the animal be emotionally effected if a human were to be harmed? I'm not justifying the harming of animals, but it shows why most humans are against harming dogs and other humans verses every other animal.