From what i read from later in the novel, it's good that almus didn't kill him here, if he did then he'd have probably had a difficult time dealing with the rigur family later on as a successor would gain power still and he couldn't justify taking out the rigur family wholesale, with this after almus successfully wins a war with a enemy nation and is effectively crowned king, he has justification to wipe out those families that opposed him openly, aka rigur's (though to be fair, it's a pretty brutal slaughter in the novel, though the justification is a legitimate one with plenty of examples in history, it might be a difficult one for some people)