In Streptococcus mutans, ComX, an alternative sigma factor, drives the transcription of the ‘late-competence genes’ required for genetic transformation. ComX activity is modulated by inputs from two signaling pathways, ComDE and ComRS, that respond to the competence-stimulating peptide (CSP) and the SigX-inducing peptide (XIP), respectively. In particular, the comRS, encoding the ComR regulatory protein and the ComS precursor to XIP, functions as the proximal regulatory system for ComX activation. Here, we investigated the individual and combinatorial effects of CSP and XIP on genetic transformation and cell killing of S. mutans. Our transformation results confirm the recent reports by Mashburn-Warren et al. and Desai et al. that XIP functions optimally in a chemically defined medium, whereas its activity is inhibited when cells are grown in complex medium. Using tandem mass spectrometry (MS/MS) fragmentation, a drastic reduction in XIP levels in ComX-deficient cultures were observed, suggesting a ComX-mediated positive feedback mechanism for XIP synthesis. Our evaluation of cell viability in the presence of 10 μM XIP resulted in killing nearly 82% of the population. The killing activity was shown to be dependent on the presence of comR/S and comX. These results suggest a novel role for XIP as a compelling effector of cell death. This is the first report that demonstrates a role for XIP in cell killing.