Abusers kill their wives and then themselves because once she is dead and motionless on the floor, he realizes that the life spark was hers and hers alone. In killing her, he sought to absorb her; now that All is gone from her, he realizes he will never ever and had never ever been able to use her life spark for his own.
Overwhelmed with true grief because he feels unable to live without leaching from her, he turns the weapon upon himself.
He truly loved her life force. His mistake was thinking he could ever take it from her and use it as fuel for himself.
See Pride and Greed to understand more about how domestic violence murder-suicides come about.