The protagonist is reincarnated into a baby (maybe toddler) in a novel (maybe game). Her entire family is evil. Since she doesn't see any possibility of reforming them, she kills her whole family at 2 years old using poison. I think I remember a strict old man becoming her guardian. I believe she spends her childhood learning the sword and training with her guards. Pretty sure there is no harem and up to where I read there was no romance either. I thought the title was Akuyaku Reijou... something or other but I can't find it.