Why'd Doris kill herself??

I have a few theories.
She realized big Dave was a fraud after having an affair w/him, feeling like a fool and was dissatisfied w/her life and he was her potential way out. She was also not going to get a promotion now that he was out of the picture. She was probably an alcoholic too. She also cooked the books at Nirdlingers and was eventually going to have to answer for that.

Ed tried to take the fall for her, but Reidenshneider's mouth ruined that. This could have been her way of paying him back and feeling bad for cheating on him. She either thought he was going to claim it was a murder of passion (could have gotten them all off the hook in a case of infidelity) or that he actually was legitimately telling the truth.

The money was drying up to defend her case and public defender was useless. Her brother mortgaged his house/business and they were "working for the bank". Slaves in other words. Her brother put family first. She despised family.

I forgot until re-watching this weekend. She was pregnant with big Dave's kid. I guess she couldn't live with herself and confessing that to Ed and then who was going to care for the kid while she's in prison? She also despised family and probably didn't want kids regardless.

Seeing her brother's and Ed's selflessness (or perceived in Ed's case), she couldn't live with herself and didn't want to burden them any longer and couldn't bear to tell/live with the whole truth.