In my understanding, which is imperfect, in a Buddhist understanding of reality, there is no absolute moral ground to find or be found on, no inherent "way of life" to live up to or learn, no true measure of worth or worthiness, no true absolute bar or standard of what is to be considered inherently good or bad, no central value judgement system, and therefore nobody to communicate that system to us from some sort of divine moral source.
It is up to each individual to discover first hand what is beneficial, what is wholesome, what is skillful, what is good. We decide what that measure should be, and set it simply based off of experienced reality: good is what liberates us and alleviates the experience of suffering, bad is what confines and deludes us, that which increases the experience of suffering. Good and bad, therefore, can be inferentially equated to ignorance vs. awareness of what our true nature is really all about. This can be brought back to an awareness of the lack of inherent good or bad value.
So, good is that which is in line with this awareness of a lack of inherent goodness, and bad can be thought of in terms of ignorance, or the deluded notion that things have an absolute positive or negative value. This is tied into relativism, into emptiness via dependent origination, to interdependency, to what Thich Nhat Hahn calls interbeing. We have a good and bad, but it is merely a convention, in no way absolute. We can say that, as long as we have apparent existences, causing pain and suffering to others merely furthers our own misery, and misery in general, and ought to therefore be considered a "bad" or non-beneficial action.
If you know, and by "know" I mean like how you know that breathing is good and sleep is nice, what your true nature is, then you will understand on an intuitive level how to free yourself from and avoid ultimate suffering, and what is the absolute extinguishment of all suffering? :) This is the same as if you know that a fire burns from first hand experience, you call the act of sticking your hand in a fire bad, due to the effect that that will have: suffering.
This happens, clearly, because certain causes have certain effects. For instance, if you commit a horrible act to someone, you will eventually come to grips with the notion that this was a human, like you, vulnerable and suffering the vicissitudes of life. This will bring about the experience of suffering, and will cause you to either become numb or more sensitive. It must be contended with in some way. This is a form of suffering.
More so, you were suffering before you committed the act that led to further suffering. In other words, you had a feeling of suffering that acted as the motivation behind an act. That intention/motivation/feeling of suffering becomes stronger when you act on it, though we are seeking to satisfy ourselves. So it is understood that, ultimately, it is this intention that defines an act as having moral value, it is intention that creates good or bad karma, also perception, also effect. However, in Buddhism, the "aim" is to free yourself of karmic conditioned existence. It is seen that even "good" karmas are the causes of suffering, and are just a part of the truly good or truly bad trap.
So, are your actions set with the intention of extinguishing suffering? If so, with a correct understanding of what causes suffering, then they might be beyond karmic consequence. Are they set with the notion of true existence and value? If so, they will likely act as seeds, as cause for immediate or future suffering, and are ultimately non-beneficial towards the goal of ultimate, final liberation.
Question must be in the "why" of things, and have room for weather or not the apparent actor is aware of the consequences or effects of the actions that are being committed in the name of said intention. So, is there a killing where the intention was truly beyond the perception of absolute good or bad, motivated purely by compassion and free from all misapprehension and motivational suffering? I can't answer that for you, but I cannot imagine myself in a situation where a killing would not lead to more suffering. What decided that? I would argue just the bare, basic, un-elaborated, simple ole truth of our lives.
Remember, positive and negative are subject qualifications and are based on pleasure and pain, which are also subjective. We cannot value things in any absolute way given those truths. The universe does not judge you, you do a fine enough job doing that yourself.
There's a great story, and I will have to find the sutra this comes from, where a man came to the Buddha asking him to help, there was a dire situation. This man's brother (or some relative) had died, and they had not performed the necessary rights to ensure a safe passage to a heavenly afterlife, or good rebirth. He knew of the Buddha's fame as a sage and teacher, so thus sought out his expertise, certain that the great Buddha could alleviate this misfortune.
The Buddha instructed the man to go get two very light weight "jars", and fill one with melted churned butter, and the other with rocks, and bring them, with the body, to the river. Excited, the man did this, believing the Buddha would surely help his brother.
When he came back, the Buddha was waiting for him, and instructed him to place both jars in the river, and when he did so, they just floated there, slowly drifting away. The Buddha then broke open the first jar, and the butter stayed on top of the river as the jar broke. It frothed up, and was carried away. Then he broke the other jar, with the rocks, and the rocks all sank down after the jar was broken.
He told his new friend that there is nothing that anyone can do after the life is over. Only the weight of our good or bad deeds determine where we go in rebirth, as it does so from moment to moment in this life.
Our actions leave impressions in our consciousness, in our minds and on our hearts, based on our own sense of value judgements, which are based on our own sense of what is truly real and existent. These impressions carry us away, or sink us down. How is it not the case?
The trick is transcendence, to develop an awareness of the true nature of reality (anatta, anicca, dukkha) and get your intention fixed in a alignment with it. This is done by following the 8 fold path, and the morality section is based off of simply making it so that there are no mental distractions that will keep you from developing your awareness. As such, they are based off of non-attached, dispassionate, compassionate social activity. They are aimed at selflessness.
Karma is based off of volitional reactive activity to perception driven feelings, the quality of those feelings, and our relationship to them in terms of either our tolerance to them, or their influence over us. To reduce blind volitional reactivity, we can develop awareness and equanimity via meditation.