The YouTube shooter told cops she wasn't planning to hurt anyone hours before she continued her trip to carry out the attack ... according to new police body cam footage.

Nasim Aghdam's seen calmly talking to Mountain View, CA cops in the early morning hours of April 3. She explains why she left her family in San Diego before answering "No" to a series of questions about whether she wants to hurt herself or anyone else.

It's pretty eerie ... the cops are half-joking around with Aghdam by the end of their talk before deciding to let her go. Hours later, she shot and injured 3 people at the YouTube campus before killing herself.

As we reported ... Aghdam's father says he reported her missing in Southern California before her run-in with Mountain View cops, and when they contacted him they said they found her and everything was "under control."