It's an event we know pretty much nothing about.
1. Do you think she would do it for the evil lulz?
2. Do you think it would fit her character writing in BG2?
Not just for kicks, but there are a number of reasons why she'd kill a family.
And sure it fits.
Mazzy: Listen to me, drow. Though our leader has made the foolish choice of allowing you to join this party, you shall receive no welcome from me. Slayer of innocents! You taint the sun with your evil!
Viconia: Ha! You do not frighten me, tiny pretend-paladin. I've had taller footstools and am little concerned with whatever brawn your diminutive form could muster.
Mazzy: Cruel words will do you little good when your head is on a pike. Step across the line even once, and I will kill you.
Viconia: We shall see when Fate plays her hand who shall be the last one standing.
Viconia: Sarevok, may I share a whispered word with you?
Sarevok: Yes, drow? What is it you want?
Viconia: I miss the customs of my homeland, like the breaking in of new pleasure slaves. The largest and strongest were the hardest to break, but they were the most rewarding. Sarevok, I find your great size... intriguing.
Sarevok: Were you to break me, Viconia, you might find nothing but the chill emptiness of the grave within.
Viconia: That does not repulse me as you might imagine. We drow are ever eager to broaden our... experiences.
Sarevok: I orchestrated a war to slaughter thousands. I have felt the cold embrace of death. I have witnessed the horrors of the Abyss. But you, Viconia... you scare me.
Viconia: Cowards everywhere I turn! If you find your courage, Sarevok, seek me out.
Viconia: Sarevok, have you had the opportunity to consider my earlier offer? Untold pleasures await you if you would but submit to me.
Sarevok: I have considered it, Viconia. And I must reject you and your... temptations.
Viconia: A pity your manhood is so lacking. I am a cruel mistress, but my slaves always found their subjugation to be most satisfying to their own physical desires as well.
Sarevok: When Bhaal held sway over my soul, I reveled in the bloody carnage I wrought. But my will was not my own. As captivating as your dominance might be, Viconia, I will not surrender my being to the whims of another again... be they god or drow.
Viconia: Then you are of no further interest to me, rivvil... though I suspect your dreams will be filled with dark imaginings of the hedonistic pleasures you have denied yourself. But even your dreams will be but a pale shadow of my true decadence.
Sarevok: Of that, Viconia, I have no doubt.