There are plenty of manga that depicts serial killers skinning their victims alive, making human furniture, mutilation, and other ways of torture. Eating a human to absorb their skills is, in hindsight, more practical than eating mindless monsters.
After eating 21 slimes, she should be level 22, right?
And she became level 24 after eating "several" steaks.
But since she gained 1 level per monster, that meant she only ate 2 wolf steaks.