The 'retributory' version of karma would assert that when you kill for arbitrary reasons, you'll get killed for arbitrary reasons.
The more classical version of karma, based on 'habitual tendencies', would reach the same conclusion… not because of some universal retribution / justice system or another balancing mechanism, but simply because arbitrary killing is the very behaviour you promote although it is manifested in the world you live in! As you're not 'above' the world you create, as you co-dependently arise with it, the seeds you plant will affect you: you won't be in some 'separate' place when the fruit arises… If you promote tendencies of selfish arbitrariness, living in a world full of selfish arbitrariness (not just your 'own') might prove unpleasant!
There is no other mind for you to appropriate, to experience the world differently, to experience another world. If you wish to cease the unsatisfactoriness of life, cleanse your 'own' mind and realise selflessness ('own'-less-ness).
Realising emptiness, one sees phenomena as arising and ceasing processes, not just as entities, fixed qualities, certainties… "Form is emptiness": seeds are forms, seeds are a step in a process, seeds are not separate from the process…
When you kill for arbitrary reasons, you'll get killed for arbitrary reasons. But who might kill you? When the 'empty' process takes 'form' as fruit, do you blame the killer, or do you take responsibility for setting up the motion, for giving rise to the process?
While "samurai Zen" had at times some rather unexpected conclusions (e.g. justifying killing, without seeing a contradiction with the first precept), these conclusions were not as disconnected from wisdom as it might superficially seems. The pedagogical point remained about understanding causality, and about how we shape our experience of the world by ignorance of its true nature (e.g. pretending one won't get caught, or that holding a rifle allows one to safely bully others).
The form the teachings take might be surprising, but form is empty! Do you take them as opportunities to enquire, or as self-serving reassuring certainties to cling to?
video excerpt (2'21'') from Ghost Dog: The Way of the Samurai, by Jim Jarmusch, with Forest Whitaker (1999)