Remember, Steve is pretty depressed after he learns that Judy has taken his fortune. He's wanted by the cops for money laundering, he's an accessory to the hit-and-run murder of Jen's husband, and the authorities are onto him. Steve has lost several pregnancies with Judy, which, though he's a jerk, is a personal crisis that would take its toll on any human. Judy and Steve even split up amid their problems. Then, in the finale, Jen rejects Steve's misguided advances. Maybe this is all too much for him and Jen isn't the one who pulls the trigger after all.
We learn through a series of flashbacks that Steve wrestles the gun away from Jen. Maybe he turns it on her for a moment, but then, the gravity of what he has done finally hits him. He killed another man! He turns the gun on himself, and Judy is left with fresh grief, since we know, she always kinda wanted to get back together with Steve. She is also now the only person who can pay for Ted's murder. Will she finally turn herself in?