She offended by comparing herself to Leto and saying that she was BETTER than Leto. This offended Apollo and Artemis because she made a big deal of having more children than Leto. The gods then killed her children as punishment. (In some stories they spared one boy and one girl each).
Why is this important? Motherhood is unique to each mother and NO mother is “better” than another. They are just different. Each mother is special and no mother should be made to feel that she is not a worthy person…….