Collective fear stimulates herd instinct, and tends to produce ferocity toward those who are not regarded as members of the herd.

Russell is describing a recurring social-psychological mechanism: when fear becomes shared and amplified within a group, individuals tend to surrender independent judgment to “herd” belonging. That collective anxiety sharpens boundaries between “us” and “them,” making outsiders appear not merely different but dangerous, and thus legitimizing hostility. The remark fits Russell’s broader concerns about propaganda, nationalism, and mass emotion in modern politics—how irrational passions can override liberal tolerance and produce persecution. It also implies a moral warning: resisting cruelty requires resisting fear-driven conformity and maintaining a sense of common humanity beyond group membership.