This question got me thinking about Theresa's death.
In season 1, episode 6, "Trompe L'Oeil", Ford orders Bernard (now revealed as a robot) to kill Theresa. This scene shows Ford can control the robots very easily with the right spoken commands from him.
Theresa is revealed to be a robot herself in a memory scene with Bernard. She and Bernard were talking in bed when she suddenly went motionless for a few seconds. Tellingly, their conversation was about robots talking among themselves even when no humans are present.
If Robert Ford can control robots by speaking the right words, he should have been able to control both Bernard and Theresa.
No need for the revealing conversation before he orders Bernard to kill her. He could have just told them both to go into analysis mode, or freeze all motor functions, or reveal their secrets to him. He did not do any of those actions.
Why did Ford command Bernard to kill Theresa?
Her death causes several problems for Ford. He also has to command Bernard to "clean up the mess". He has to come up with a plausible explanation for her death for the human employees. He has lost a valuable asset of a robot he could use as a spy within the company.
The out of universe reason is that her death looks dramatic, but I prefer an in-universe explanation.