This paper investigates how mathematics teachers describe their ethical decision making related to instructional practices, drawing on frameworks that incorporate both rational and non-rational approaches. We employed a multiple-case study method, selecting three teachers as cases through criterion sampling. Data were collected via four consecutive in-depth semi-structured interviews across twelve scenarios designed to elicit teachers' ethical evaluations. The data were analysed through cross-case synthesis in two stages: first, a deductive content analysis of each case, followed by a comparative analysis across cases. Five key themes emerged: (1) the use of ethical and technical lenses, with technical considerations sometimes overshadowing ethical aspects; (2) the utilization of rational approaches, predominantly consequentialist and deontological; (3) the use of non-rational approaches, notably experience and obedience; (4) the coexistence of multiple rational approaches, which can lead to conflicts resolved either by the dominance of one approach or by the reconciliation of different perspectives; and (5) the concurrent emergence of rational and non-rational approaches, with non-rational evaluations potentially arising from a rational framework. We discuss the role of mathematics and its teaching/learning in teachers' ethical evaluations, the complexities of different decision-making approaches, and their co-emergence. Our findings provide foundational perspectives for developing an ethical decision-making model in mathematics education, which we outline and share.