Heaven and Earth are not ruthful;
To them the Ten Thousand Things are but as straw dogs.
— Tao-te-ching. Trans. Arthur Waley
The Ten Thousand Things: all life forms. Straw dogs: sacrificial tokens. Ruthful, think opposite to ruthless.
This is where John Gray’s book ‘Straw Dogs, gets it title. It is also I’m fairly sure the origin of the title of the film(s).