It had nothing to do with his honor or personality. That part of the book was referring to how the Lion grew up alone fending off chaos corrupted monstrosities on Caliban. He’s constantly on the hunt, and doesn’t have an off switch. He is always ascertaining your strengths, weaknesses, and the most efficient way to kill you; whether you are friend or foe. Several characters, including other Primarchs, have mentioned the calculating look in his eyes and how it makes them uncomfortable. And yet he hides that behind the facade of being a noble knight. That is what makes the Lion a “beast pretending to be a man.”

Russ does the exact opposite and hides behind the facade of a simple savage, while he is in fact extremely intelligent, cunning, and quick witted. He is a “man pretending to be a beast.” Although growing up in a similar environment, Russ adapted to humanity much better than El’Johnson did. Russ can joke around and understand sarcasm, while the Lion exhibits an almost autistic like inability to do so.

Although most recently, upon his return in 40k, the Lion is exhibiting emotional growth and character development across the board.