Sexually selected weapons used in intra-sexual competition for mates are among the most striking animal features, but how their evolution affects life history traits closely correlated with fitness is not well understood. Here, we selected for or against the prevalence of a lethal weapon in a male-dimorphic mite Sancassania berlesei, and investigated how life-histories evolve in populations with high vs. low proportion of weaponized, aggressive males called fighters and non-weaponized, non-aggressive males called scramblers. After 25 generations of selection, females from fighter-selected lines showed higher early fecundity compared to females from lines selected for non-aggressive scrambler males. Furthermore, while fighter males took longer to reach adult stage compared to scrambler males in general, both the morphs and sexes matured earlier in fighter-selected lines compared to scrambler-selected ones. Larvae-to-adult survivorship was not affected by such selection treatment. Finally, we investigated whether adult survivorship under temperature stress was influenced by such selection treatment, and we found no difference between fighter-selected and scrambler-selected lines. Our results demonstrate that selection for male weapon and/or consequent prevalence of costly intra-sexual aggression leads to an increase in key components of fitness, with likely consequences for population dynamics. However, we found no evidence that the response to such selection affects how individuals cope with environmental challenge.