First off, the MC chose to be transmigrated as the villainess, and now that she is the villainess, she intends to play the role as best she can in order to make her favorite characters stand out.
She was framed as a villainess and died in her past life, so when she reincarnates/goes back in time the MC decides to be the real villainess this time.
"In order to make the love of the captured target and heroine more exciting than the game, I will do my best to be mean to the heroine, but for some reason the love doesn't seem to be progressing..." -- Baka-Updates
After going back in time, the FL aims to surpass and suppress her scheming half-sister.
The male lead offers the female lead money to act as his villainous wife in order to get people to leave him alone.
The ML wants to marry a villainess, so the FL acts as one.
Wellmy is a commoner who became a noble after her mother married an Earl. She acts horribly and bullies her sister. But this is actually all part of her plan to make her sister happy, even at her own expense.
Alicia is an illegitimate child and was always treated terribly. After dying, she wakes up in the past with a ring that gives her the ability to read minds. Alicia is determined to get her revenge by living as a villainess.
Becoming the Villainess: Working Behind the Scenes to Make My Bias Shine