Through a series of flashbacks, we see Vanya struggle to develop her powers, gradually becoming more and more powerful until she begins to realize her full potential. Reginald's response is to lock the young girl up in solitary confinement before having Allison brainwash her into thinking she was powerless. Allison doesn't understand what she's done at the time, and doesn't realize her part in things until the current day.
Allison confesses to Vanya about what she did as a child at possibly the worst conceivable moment, causing Vanya to become enraged at her just as she's figured out how to use her powers. Allison attempts to brainwash Vanya again out of desperation, but Vanya lashes out at her, slashing Allison's throat.
Leonard's reaction to walking in to Vanya crying over her mortally wounded sister? He grins.
As Vanya apologizes to Luther about hurting Allison, he seems to be understanding and pulls her in for a hug. However, as he starts to slowly crush her, Vanya begins to squirm and panic, trying to make Luther let her go until she falls unconscious.
Vanya calmly going through her Roaring Rampage of Revenge, destroying the whole mansion as she walks by and even coldly murders Pogo after he reveals he played a part in what made Vanya who she was today.