Spodoptera frugiperda (fall armyworm) has been a severe maize pest worldwide, and it is essential to study the mechanism of its outbreak. Compared to other lepidopterans, Spodoptera frugiperda larvae show higher aggression toward competing individuals. Although Spodoptera frugiperda larvae are predominantly herbivores, they also exhibit cannibalism by feeding on conspecific larvae and act as intraguild predators by preying on the larvae of maize stemborer species. Cannibalism and intraguild predation (IGP) are typically seen as advantageous for consumer populations. While this is straightforward when consuming prey improves individual performance, the overall population impact of cannibalism becomes less clear when it leads to a decline in individual fitness, including lower development speed and survival rate, as observed in Spodoptera frugiperda systems. While cannibalism negatively impacts Spodoptera frugiperda, the effect of IGP depends on the specific species of maize stemborer being consumed. In this study, we developed a stage-structured consumer-resource model with two species, Spodoptera frugiperda and maize stemborer, where Spodoptera frugiperda simultaneously engages in cannibalism, intraguild predation (IGP), and resource competition. In a single-species scenario, we show that aggression reduces the population size of Spodoptera frugiperda across varying levels of positive or negative impacts of cannibalism. In the two-species system, regardless of the effects of consuming intraguild prey, the negative individual-level impact of cannibalism instead leads to an increase in the population size of Spodoptera frugiperda. This occurs due to the overcompensating positive effect of reduced resource competition with maize stemborer. We also investigate how these behaviors influence community dynamics when the maize stemborer species has superior competitive traits, such as reproduction, resource consumption, and survival rates, under resource competition. In these cases, Spodoptera frugiperda must exhibit stronger IGP against the maize stemborer to avoid extinction. We propose that this pattern explains the necessity of non-selective aggression while confronting a stronger resource competitor -- instead of the nutrition values gained by consuming the intraguild prey, the resource released following competitor death is what makes aggression a beneficial competition strategy for Spodoptera frugiperda. Our study emphasizes the importance of resource competition and community context when looking into species-level aggressive behavior.