Togas introduction had her killing what looked like a kid by draining his blood. Her backstory is that she feels a compulsion to drink peoples blood she likes and she can't really control that urge.
I know hori tried to sell the idea over time that it's societies fault for not accepting her that she's like that. But I never bought it. Maybe if it was done in reverse, where she's empathetic at first but then becomes more of a mass murderer over time. The way it was done, just felt like retroactive "sympathetic villain flashback time