Pneumococci (Streptococcus pneumoniae) aren't the most affable of bacteria. They make antimicrobial agents such as bacteriocins and bacteriolysins that are directed not, as is the usual case, against other species, but against their own. This phenomenon, dubbed fratricide has been linked to the competence status of the bacteria, that is, to the ability of the bacteria to take up extracellular DNA. The killers are competent cells, and those that get killed are ones that have not become competent.
In a recent review, Claverys, Martin and Håvarstein discuss several facets of streptococcal fratricide, concluding that it may well be widespread among the streptococci. Do you want a teleological explanation? The authors speculate that fratricide may be not only a way to increase genetic exchange but also a mechanism for releasing virulence factors that may enhance the pathogenicity of virulent strains.