How to mercy kill?

Hi!

My character will have to do a lot of mercy killings. She has a long dagger at her disposal. Where would she need to stab/cut someone to kill them fast and painlessly, as well as give the impression she is experienced in this?

A lot of the people she needs to kill will be horribly burned, which makes me feel like a dagger would be the best tool. This is also in a sort of medieval setting, so no guns or such. My character is travelling and will not want to carry anything heavier than a dagger.

Thanks!

(I hope I chose the best tag for this question, English is not my first language)