I actually think rather than ever planning revenge, she was always grateful to him from the very get-go for killing her father.
Before the father starter raping girls at school, I bet he first started out by abusing his own daughter. Shika could've endured years of abuse at his hands before...
Oh damn, this is actually a really good depiction of autism (my younger brother has aspergers). Never thought I'd see it so accurately conveyed in a manga; for those that don't know, her inability to properly empathise with others is pathological - she's not being malicious, she genuinely...