Keith Wiens testified the couple got into several arguments on the night of the murder in August 2011, saying Lynn Kalmring suggested he didn't love her because she was too old and overweight.
Wiens told the jury Kalmring came in and out of their bedroom several times, at one point spitting at him and turning purple.
The 57-year-old says it was the worst confrontation he'd ever had, even as a police officer.
Wiens says when Kalmring again left the bedroom he got out his gun.
He says she returned with a knife and he shot her in the face. (CKFR)