PurposeThis paper aims to examine the effect of product market competition on firms’ unethical behavior (FUB) in the Chinese insurance industry and to further explore the boundary conditions of the main effects. On the basis of China’s commercial foundation, the study constructs a conceptual framework of FUB by drawing from the perspective of horizontal competition.Design/methodology/approachData were collected from 52 property insurance firms at the branch level observed over the six-year period, 2011-2016. Within this framework, market power and market concentration were used to describe product market competition at firm and industry levels, respectively. The moderating effect of market munificence was analyzed to reveal the theoretical boundaries of the main effect. By drawing upon cost–benefit analysis and social network theory, the study used negative binomial model and Poisson model to quantitatively examine the relationship.FindingsThe relationship between product market competition and FUB is curvilinear. Especially at the firm level, market power exhibits a U-shape relationship with FUB; at the industry level, market concentration exhibits a U-shape relationship with FUB. In addition, market munificence positively moderates the impact of firm’s market power on FUB, whereas, market munificence negatively moderates the impact of industrial market concentration on FUB.Research limitations/implicationsThis paper explored a new type of unethical behavior that concerns consumers or the third party by emphasizing horizontal competitive contexts; it also provides a better understanding of the FUB–financial performance relationship from the perspective of competition. The moderating effects suggest that when the cause of FUB is different (market power vs market concentration), firms may make opposite ethical choice. However, the sample is from a single industry; it will be fruitful to further verify these findings in other industries such as the manufacturing sector. Moreover, the definition of FUB is confined to explicit forms such as participation or collusion but there is no way to measure the implicit forms of FUB.Practical implicationsFirst, the governance of FUB should not only focus on the firms themselves, but also take into account the industrial market structure. Second, proper use of governance measures for FUB can increase firms’ benefits from “compliance with the law”, enticing firms to decrease FUB. The third, firms with weak market positions, facing fierce competition, should not be involved in FUB for short-term benefit; indeed, a low-cost strategy can be adopted as the dominant competitive strategy. While, in cases of highly concentrated market structure, firms should strive to avoid involvement in FUB through collusion with other rivals.Social implicationsAs it is a very common phenomenon that firms in competitive relationships may adopt FUB toward third parties or consumers, this trend has become a hot topic in the economic and social development in China. The study’s conclusions reveal that a more proactive and ambitious ethical decision is desirable for all kinds of firms; moreover, firms should make a rational choice between “short-term interest” and “long-term survival”. When firms identify the compliance of business ethics as an opportunity to differentiate themselves and perceive the benefits of decreasing FUB as outweighing the costs, the level of FUB will be inhibited, and social welfare will increase.Originality/valueThe primary contribution of this research resides in identifying product market competition as a previously unexplored predictor of FUB, thus revealing the dark side of product market competition. In addition, nonlinear relationships between product market competition and FUB indicate that situations of competition exert an important influence on FUB both at the firm and industry level. This paper’s conclusion provides a more meticulous theoretical explanation for FUB. This research demonstrates that the traditional ethical framework is not sufficient to explain FUB in a horizontal competitive context. Indeed, resource constraints and competitive pressures should also be considered.