He was probably going to torture him. No reason to keep him alive otherwise.
The Wiki page is confusing. In one line they say the girl’s stepfather was the German Doctor accused of killing her.
Elsewhere, this is contradicted, but never corrected.
The coroner’s report shows that the ‘girls’ genitals were removed and have never been found’. I didn’t know that was possible with a female without leaving a very conspicuous open cavity in the front of the body.