There is no grey. Grey is an admixture of black and white, taken from a general, non-specific viewpoint. As one moves closer to the seemingly grey picture and begins to uncover details, it is shown to be a series of exclusively black and exclusively white shapes forming the mass which gives the illusion of grey.
Killing is grey because there is no detail in just “killing”. As one begins adding detail, one begins to focus down on the picture and eventually, once all details are known, there will be no grey but pure black or pure white.
God, who knows all details, does not see any grey at all.