Keeper of the Gate of Grudges presents each soul with three choices: pass through the Gate of Grudges to heaven and be reborn refuse to admit death and remain as a ghost on earth or haunt and kill one person and then proceed to hell and eternal suffering. Izuko was a stillborn baby so she arrived at the gate without even knowing which option she would choose. In the end the superiors not cleary if the superiors are demons gods or etc decided to mantain her as the gate keeper. The only rule is that she cant choose the option for the person or she would fall in hell.