I think that the call for mechanics to be used is in relation to the situation of them being imprisoned/detained; what happens next should be the result of the game mechanics. Essentially, if there is any doubt about the outcome, then let the dice decide in some way.
The way I see this, killing these PCs out of hand doesn’t offer much benefit, does it? Teaching the players a lesson of some sort seems to be the case?
Seems far more interesting stuff might come from following these events and continuing along this new path.