Biomembranes are involved in diverse cellular activities. How membranes and proteins interact in the activities might hinge on the former's physical characteristics, which in turn are influenced by packing of lipid molecules. Yet, the validity of this understanding and its mechanism are unclear. By varying chain saturation of membranes, we explored correlations between lipid packing and peptide-mediated membrane disruption for the antimicrobial peptide, melittin, and amyloidogenic peptide, β-amyloid (1−42). Remarkably, reducing molecular packing flexibility enhanced the membrane disruption, possibly due to a shift from membrane perforation to micellization. A theoretical analysis suggested the energetic basis of this shift. This mechanistically shows that a peptide's mechanism might be dictated not only by its intrinsic properties but also by physical characteristics of membranes.