This is the largest mistake made in the series. The Duel was to the death and MC changed it so that it was not. If he kept the Duel to the death as it should have been, then Cleave Die would have not died. It was the defeated guy that stabbed Cleave somehow even through both Cleave and Nikola are both extremely quick and should have been able to stop the stab. The Duel opponent gave a mortal stab to the MC even through the duel was not to the death. It was only the MC's stupidity which should have been over with already (about killing fools), that let him live. It doesnt make sense, since he has been killing people for less since day one.
People aren't unchanging so it could be something as simple as his mood/heart not being in it and frankly a duel is a very different type of battle to unorganized figthing between adventurers, bandits or monsters. He might have though that since it was a duel it made sense not to kill, thereas in a less formal setting he would have killed him without thought. I also don't think this was just som random thug, it was someone with influence.
