You make good points - and I have therefore changed the game thread to read that Marron strikes the killing blow.
(In particular, I should not have made such a decision on Walker's behalf - sorry.)
Marron simply had a rush of blood to the head - although the Empire Soldiers were hard to hit, he should have consulted the party.
(N.B. Empire soldiers are trained in a 'cult-like' fashion to fight to the death against inferior non-Empire opponents...)
Please feel free to change your game thread posts to react to Marron's action (i.e. NOT Walker's.)
P.S. Once you've had a chance to do that, I'll post Marron's considered thoughts on that final slaying and what he suggests next...