Host‐defense peptides (HDPs) feature evolution‐tested potency against life‐threatening pathogens. While piscidin 1 (p1) and piscidin 3 (p3) are homologous and potent fish HDPs, only p1 is strongly membranolytic. Here, we hypothesize that another mechanism imparts p3 strong potency. We demonstrate that the N‐termini of both peptides coordinate Cu²⁺ and p3‐Cu cleaves isolated DNA at a rate on par with free Cu²⁺ but significantly faster than p1‐Cu. On planktonic bacteria, p1 is more antimicrobial but only p3 features copper‐dependent DNA cleavage. On biofilms and persister cells, p3‐Cu is more active than p1‐Cu, commensurate with stronger peptide‐induced DNA damage. Molecular dynamics and NMR show that more DNA‐peptide interactions exist with p3 than p1, and the peptides adopt conformations simultaneously poised for metal‐ and DNA‐binding. These results generate several important conclusions. First, homologous HDPs cannot be assumed to have identical mechanisms since p1 and p3 eradicate bacteria through distinct relative contributions of membrane and DNA‐disruptive effects. Second, the nuclease and membrane activities of p1 and p3 show that naturally occurring HDPs can inflict not only physicochemical but also covalent damage. Third, strong nuclease activity is essential for biofilm and persister cell eradication, as shown by p3, the homolog more specific toward bacteria and more expressed in vascularized tissues. Fourth, p3 combines several physicochemical properties (e.g., Amino Terminal Copper and Nickel binding motif; numerous arginines; moderate hydrophobicity) that confer low membranolytic effects, robust copper‐scavenging capability, strong interactions with DNA, and fast nuclease activity. This new knowledge could help design novel therapeutics active against hard‐to‐treat persister cells and biofilms.