Yeah, it is a challenge to gain the mental strength to end a character, let alone a sibling of the main cast to prevent a catastrophe. But let's be for real, would you seriously risk experiencing a nation-wide disaster over a few feelings? The time window for him to find any semblance of a cure or nullification has long past. If anything, he's just wasting time. I personally think he's too invested in the characters that he forgot about the looming threat coming soon.
All he has to do, is run into the room, put two in the head and leave no traces. Problem solved.....BUT if he does do that, there is a chance the co-author will place a second disaster as stated way earlier in the story. But who knows, maybe the author will forget that plot point and make it a plot hole. Hey, it happened before in 100's of manga and manhwa, it might happen again. Who knows?