There is that shared sense in Russ and Lion, that they don't care what people think of them, they just do what is necessary.
Russ even seems to play with people preconception of him as a brutal savage to throw people of guard.
I suspect that the Lion's dislike for Guilliman, is an intrinsic dislike for the Politician. People who do care more about what others think of them, instead of unquestionable Loyalty.
Would be interesting to see the play between these Lion and Russ on the rush to earth.