Kiriko literally said she killed 150k+ people. She's partly trying to atone for her sins and ensuring that nobody else gets harmed through her experiments (ironic though considering if she fails mankind probably will). Seems a little silly for sure, but I think it's consistent with her character. Was there rationale on why she couldn't use it on animals?
Also she really reminds me of Hohenheim from FMAB. Really like her character.