The antonyms for "become lenient" include become strict, become harsh, become rigid, and become inflexible. While leniency is often seen as a positive trait, there are times when strictness and rigidity are necessary. For instance, a teacher who is too lenient may struggle to maintain discipline in the classroom. Similarly, a judge who is too lenient may fail to deter future crimes. In contrast, strictness and inflexibility can be useful in situations where following strict rules is necessary, such as in fields like medicine or engineering. Ultimately, the appropriate approach depends on the situation and the goals being pursued.