>>I had a green card on the way when I quit my job so why wouldn't I just stay if I wanted to immigrate so badly?
This phrase caught my eye. Based on this, you probably thought that you could leave and enter the US at will. Ironically, during this phase of the green card process (i.e. right after the I-485 is filed and awaiting final processing for denial or a GC in the mail), my understanding is that you'll have more difficulty entering and leaving the country without special dispensation (called Advance Parole).
Assuming that your company lawyers handled your green card application, I'm surprised they didn't apply for Advance Parole for you at the same time as filing the I-485. That would have saved you a bunch of hassle, regardless of whether you quit the company or not.
IANAL and all that, but I think this explains why they were so hostile. You didn't follow their strict procedures for these types of things. Of course, they're not going to tell you this during an interrogation, but this may have helped.