if you kill a being there is karma attached to that action, that karma binds you and the being that was killed.
these karmic imprints are left on the alaya of the base conciousness of both beings which alows for future causality to take place in later lifetimes.
this could potentially include the killed being from the previous life causing harm to the killer in this life either by intention or inadvertently.
this is a primary reason why buddhist teachings include methods to purify karma.