As much as many of us criticize when OUAT sticks too closely to the Disney versions, this is a case where they would have been better off doing so.
I know nothing about Megara-of-myth, and the vast majority of the audience is probably the same, while being very familiar with the Disney version. So yeah, in this case, they should have stuck with the Disney version instead, which would avoided this utter disappointment everyone has with the character.
Because I knew nothing about her mythological origin, I looked her up…
Megara is always portrayed in the myths as the guiltless innocent who, with her children, suffers a meaningless and brutal death.
Well, that fits with how the show portrayed her.
Later portrayals of her, such as the animated Disney film Hercules (1997), depict her as a con artist who is redeemed from her difficult past through her relationship with the hero. No ancient depictions of Megara present her in this light at all. Her role in the 2014 film Hercules, while not exact, is much closer to her traditional portrayal.