Starlight is absolutely engulfed with rage. Because of her naivety, she expected Sans to actually spare her when she played the Genocide route. Still reeling from her failure to beat him last time, and having nightmares that taunt her over this, one thing is for certain. She won't be making the mistake of showing him mercy again, no matter how bad it makes her look.
It's too bad Twilight has left her uninformed about what happens AFTER Sans. Otherwise, Starlight would've let it be.
Note from Author: This will probably be a pretty short fic.
Sort of a crossover with Undertale. Contains spoilers for Undertale's Genocide route
Reading the first fic is recommended.