Time Harvest
Thalia needed to figure out Cassia's ability first. If she didn't, there was no way to make sure Cassia stayed dead.
No matter how fast a person died, there was always a short process. Even if it lasted just 0.001 seconds, someone with mental-energy powers could still have time to use their skill.
Like illusion users, or... someone able to rewind time.
All at once, Thalia figured it out.
Those kin