Its a crash course to prepare someone who has never killed a thing, much less anything human shaped.
Unless you've got a steel will or you're a psychopath, if you've never actually killed a person, zombie or otherwise, can you say for sure you won't hesitate? Especially if its someone you recognize?
Its a crash course to try and curb her hesitation, like what she tried to do when he initially went for the first strike against the mom. Hesitation means death, and if she's too mentally weak to deal with seeing her zombie mom crushed, then theres no way she could survive with or withoht people.
Its not great but its not like they have time to mentally train the girl properly. Its still early stages of total societal collapse with no safety nets. Even if MC didn't have isekai cheats it'd be just as harsh or harsher.