I speak for myself, and hoefully for at least some of mercinae (after all, I am a baron) when i speak of my system. I aspire to it, and live up to it. I encourage others to live up to it. This does not mean that everyone uses it.
I cannot comment on the case you mention, as I know nothing of it, only hearsay, which you cannot base judgment upon. I myself, kill, in return, those who kill/rob my mages, without provocation. If a mercinaen calls for my help, i help. if i see a mercinaen killed, i ask if they were ambushed/slain for no reason/etc. I then act upon this, meting out justice (i. e. a death) if they were attacked without honour or justice. My fellow barons do as they see fit, i suspect it is a similar system to my own.
In conclusion, i can only speak for myself, not others. Without hard evidence, I cannot judge someone else actions. Approach me with exact details, and i will tell you whether i agree with the action taken or not. However, I have found my fellow mercinaens to genrally be believers in justice, especially the barons. With this, i find it unlikely that they have acted with undue cause.