my interpretation of fell alphys is quite messy
I do believe no one in Underfell started out a bad person, especially alphys
with the pressure of being the royal scientist after gaster's disappearance, I imagine that she slowly became more and more deranged, and with that, came the guilt of her experiments
though her amalgamates weren't entirely an accident.
curiosity is a curse, and with her horrible doings in mind, alphys started blaming not herself, but "Dice". Dice, she claimed, was the "bad version" of herself, the one doing all these things, becoming a tyrant like the king himself
but she knew that wasn't the truth.
there is no Jekyll without a Hyde. The Hyde has to come from somewhere, Jekyll just has to embrace him. Alphys had to embrace Dice.
and when she did, all she truly did was embrace her true, inner self. Consumed by hatred, despair, fear, anger. Lacking any sort of hopes and dreams.
The doctor was always destined to lose.