Read this: Picking Our Parents and Our Life Circumstances
and this: Destiny vs. Free Will
I don't personally believe that people and animals incarnate as the other.
Animals choose to incarnate knowing that they MAY be slaughtered for food, but still do it to bring awareness and compassion to the plight of food animals.
And I do think some people incarnate knowing they will suffer terribly but do it to bring awareness to the whole of humanity, to give us the option of changing.
People who are murdered generally are not choosing their time of death. but in some circumstances, which are rare, they agree to be part of a murder situation or at least know it's a strong possibility.