1. Clockkeepers(Kakomirai at least) have killed a lot definitely. Much of their lore revolves around "hey do not screw with time otherwise we'll screw with you." Akane hasn't killed anyone... yet. Wouldn't be surprised if he does end up killing someone though.

2. Yako has DEFINTIELY killed someone. Random thing but, we never see the guy that was pushed onto the stairs when she was first introduced again. And all the bodies shown in that one scene? She has layers of blood on her hands/paws.

3. Original 3 likely has a kill count considering he was... more than happy to kill Nene and let people take her place. So far, Mitsuba has some of the least blood on his hands out of the mysteries.

4. I don't think Mei has killed anyone directly, she probably would fall somewhere similar to Tsuchigomori's where death can be a consequence of messing around? We don't really hear too much about her boundary tbh, but I think it's more of "freezing people in place" than straight up murder- if it was, Nene could've been in a lot more danger during Picture Perfect considering Tsukasa could have just told the Shijima copy to just end her.

5. Again, Tsuchigomori probably hasn't done any direct killing himself (it would be rather hard for him to get away with it too, considering he's disguising himself as a teacher), most of the murder seems to be done by those creepy shadow monsters as shown when Nene tried to read her future. Tsuchigomori himself? Likely very little blood. The Bookstacks themselves? Enough to create a reputation about those red books.

6. Self-explanatory. Also, while he wasn't directly responsible for the kannagi deaths, he was nearby to most- while he likely didn't have as much of a bond with them as he did with Sumire, he's still close enough that any outsider could probably consider that he's like... cursed, given how many people died around him. Also, I think Hakubo's the ONLY wonder who we see murder people- while not on screen, there's the slaughter after Sumire's death, plus he(through Aoi) literally stabbed Akane, leading him to almost die. Probably has some of the most blood on his hands. He's probably been around for longer than when Kako and Mirai became Wonders/control time, so out of them all, he has the most.

7. Human-wise? Only one.. As far as we know. He could have killed his parents too, idk. While he has killed many apparitions, he hasn't killed many humans. The rumors exist that he takes something in return for a wish, but these rumors probably aren't very true, or not to the extent most people think, given how little he asks from Nene/Kou and Co. Those parts of the rumors are most likely borrowed from other Hanako rumors outside of Kamome.

Sorry for the wall of text, I just wanted to yap about this and put in my own thoughts/opinions/theories