The first chapter was the attempted murder of that fool, I don't see why you thought he'd be spared. If anything, I expected him to kill himself, by accidentally drawing the attention of some fleeing orcs, or from that one gunshot, and leave Kazuhisa's hands clean (technically).
Because that's one of the abused Japanese media tropes. "Protagonist pushes through his baser needs of revenge and through his trials he achieves such growth and character maturation, he finds it in his heart to forgive those he hates the most".
I mean look at "Rising of the shield hero",(possible spoilers) throughout the entirety of the first arc the MC wants to kill Malty and the King and even through during the trial of the two at the end of the arc he literally screams for them to be executed, he "finds it in his heart to forgive them, at the request of the Queen, and chooses to mitigate their death penalty to slavery and humiliation".
A trope the Anime producers further reinforce in the Anime adaptation of "Rising of the shield hero" by having the MC and the Queen's roles in the trial be reversed, by having the Queen seek the death penalty and the MC being the one who has matured so much he no longer requires Malty and the Kings death, so he beseeches the Queen to settle with them being enslaved and humiliated.