Many micro-organisms are known to produce efficient toxic substances against conspecifics and closely related species. The widespread coexistence of killer (toxin producer) and sensitive (non-producer) strains is a puzzle calling for a theoretical explanation. Based on stochastic cellular automaton simulations and the corresponding semi-analytical configuration-field approximation models, we suggest that metapopulation dynamics offers a plausible rationale for the maintenance of polymorphism in killer-sensitive systems. A slight trade-off between toxin production and population growth rate is sufficient to maintain the regional coexistence of toxic and sensitive strains, if toxic killing is a local phenomenon restricted to small habitat patches and local populations regularly go extinct and are renewed via recolonizations from neighbouring patches. Pattern formation on the regional scale does not play a decisive part in this mechanism, but the local manner of interactions is essential.