Bane of Empires
In terms of combat ability, I would generally agree.Id say, out of all the legions and primarchs, the modus operandi of the white scars best fits that of executioner and hunter.
If i was the Emperor and wanted to sanction Russ, i would send the Khan.
However, the Khagan and his White Scars were an unknown quantity. It would have been a significant risk to trust them with such a monumental task. The Khagan (at least to his own Legion) quite openly railed against the shackles imposed on him by the Imperium and thought of the Emperor as a tyrant. In my opinion, the primary trait required for an executioner would have been utmost dedication and loyalty to the Emperor (something which the Khagan lacked).