Damen goes all berserker at the Kingsmeet. it's another one of those inexplicable things in the plot of Kings Rising. damen lays out the rules to Laurent as they walk in: anyone who commits violence in the Kingsmeet must die; Even a king who is protecting his brother and kills the attacker must die because it was in the kingsmeet. That's it. There are no exceptions, no codicils, no escape routes.
And yet--damen unsheathes his sword and slaughters.
Of course Damen will swing here. As the saying goes for writers, don't hang a gun on the wall unless you're going to use it. Don't hang an inevitably fatal law on the wall if you're not going to make it part of the plot. And Damen's anger brain has a screw loose.
And, wow, it's OK for him to walk away. 
he should never have been able to. The pertinent rules of course weren't given ahead of time, as in, whatever random King was present gets to decide whether they live or die; that was never said before. And the Regent says, oh, yeah, whatever, let him live. Which seems unlike him, even to get Laurent's cooperation. it is true that the Regent seems to prefer treason and subterfuge, but still – – 
And all those kingsmeet soldiers lie dead. That could've included Nikandros not too many years earlier.