@buecherwurmin
He's literally the son of the lord and soon to be lord himself. Who's going to question him besides his father? What crime? Execution or killing in self defense is not a crime. In many medieval and/or fantasy settings, even so much as insulting nobility constituted a death sentence on the spot. When they may as well be above the law, so who's to say murder (by anybody) was a crime there, period. For all we know, killing criminals like bandits or outlaws could have been rewarded for bounties. We don't know what all laws they have there.
And even if he didn't want to make the deaths hidden, he could make it so the slaver was killed with some trumped up charges like "he robbed and attacked us, so he dead now", or any other numerous ways he could have permanently silenced him. People in power make others disappear all the time, and it's even easier in this setting, let alone being the ones in charge of making and enforcing the laws. Besides, had the slaver not agreed to the given terms, he
was going to to simply kill him then and there, so giving him that option was a moot point anyway.