we have
enfonça un couteau sous l'aisselle as a threat:
http://books.google.com/books?id=ek_PWD_hip4C&pg=PA138&lpg=P...
Were it
dans l'aisselle it would definitely be stabbing. Since presumably they wanted their victim to live at least long enough to be raped, it probably isn't a stabbing, at least not a full-blade-length stabbing, or, if it is, then upwards into the shoulder, not horizontally into lung and heart.
I think "thrust into her armpit" is as multi-biguous as the French (personally I might interpret "underarm" as the fleshy inside of the upper arm).