There is clearly something very distressing about it, but I kind of struggle to understand why I think it's worse than killing any random NPC. Though I haven't watched the video.
I don't think murdering a woman is somehow more morally reprehensible than murdering a man, and I don't see any moral issue with murdering NPCs.
If you give people access to a simulation of this grandeur, people are going to do everything with it, and not everyone internalises violence in video games the same way.
I don't know, it's awful, and really lame, but I don't know exactly why I think it is. I haven't randomly murdered anyone in the game, but I know if I had to off a witness that happened to be a woman, I wouldn't feel anything about it.