AFAIK in (some) Buddhist religions karma and this betterment to nirvana thing is not a moral question and has nothing to do with Good or Bad. Its how things work. Karma is a kind of desire, a drive - a set of wishes that makes one piece of the Spirit be an individual soul, a john doe or whoever. Hence its the Ego that wants or "must" do and fulfil these, and Nirvana is (imho) the spirit itself, when the soul is freed from this karmic shit again. And the mind works in a way that bad feelings and thoughts are made by opposing and struggling them, fear and sorrow and all these only last as long as one fights them in a knotty way - same applies to headaches. Thats why some meditation practices are for learning how to let things go and let it flow normally, like stopping the storm, the big waves of the mind, and as you stop stirring it up, it will calm down to be a smooth, waveless see.
So in this sense, karma is like an itching that wont let ppl to rest. And if someone kills a man (which needs quite a lot of ego-istic forces, drives, hatred...), besides that its not good, it will upset his soul so much it will prey on his mind for yearhundreds.. So its mainly impractical. Thats why you can either fulfil your karma (if it can end that way, like, you feel you need to be a doctor, so be it), or learn to get rid of it some other way.
(i hope some part of my text is written in english
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