Maybe just cause he is a psychopath who genuinely likes killing people and did it randomly for the fun
I believe the glasses dude revealed he manipulated Doha (who didn't have his full memories yet) to do so to get closer to the true ending. Glasses dude wanted to try different things to see if it was possible to have more one person pass the true ending. He was able to successfully manipulate the situation for Doha to kill partly because he is also psychopathic by nature so killing others is not too far off what he is naturally capable of.
I forgot which chapter but Bada and the couple go through the forest and they encounter the same (i think the same) npc, but the npc just swings at them with a hatchet of some kind and they kill him pretty quickly. I think the villagers that show up in the game r not friendly.
okay yeah but like why'd doha kill that villager at the start of the series. still wondering about that..