the liberal objection to just killing/imprisoning/etc enemies is: that you should find some other justification for doing so, that you should not do so because morals, that doing so will be ineffective/rebound on you, or that you should generally stick to softer manipulation?
Not liberal - game theory. King sets his rules, obeys them or all his individually weaker Dukes know he'll come for them one by one. Kill people for no bright line reason and you're a danger to everyone and will be treated as such.