Ages 1-12: 0. [we can assume that during his childhood/time in the orphanage, he wasn't running around flipping out like a ninja.]
Ages 13-18: 0 [During this time he finds Kanan and adjusts into a dorkishly happy fluffy individual. Again, let's assume he's not killing people]
Ages 19-22/23: Let's break this one down:
- Kanan gets stolen, Gonou slaughters half his town. BUDDHA says as much, so you have to assume it's fairly literally true. Content: Mostly human, some youkai. The town is small enough to have only one school (though presumably multiple teachers, since Gonou gets 'hired', which implies a school with more than 1-100 students, I'd think; I attended a TINY grade school with 200 students and that had two teachers total), but large enough to have caught the attention of Hyakugan Maou. For a wild guess, based off the school percentage, let's assume that there are about 250 students total. (May be high, may be low.) Now, let's assume (based, sadly, off modern statistics since I can't find any from about 200 AD) that 25% of the population are children; Gonou specifies that 'every child' can attend that school. That'd make a total town population of about 1000, which seems perfectly acceptable for a small but not tiny town.
Kill count: Approximately 500.
- Gonou tracks down the Hyakugan Maou clan and systematically exterminates them. Chin Yisou clarifies that there were 1000 members of the clan (he clearly knew enough to try to guess what'd happen if HE spilled HIS blood on Gonou).
Kill count: 1000.
- (Kill count during his recovery period: 1. But that's peanuts)
- Burial arc kill count: Presumably around 20-50, to judge from the apparent size of the gang/size of lair. Let's say 30.
- Sanzo sends them on missions regularly. Hard to say how often, or against what, but from the looks of things they were probably against groups rather than individuals (he was surprised that the Sanbutsushin would send him after a (singluar) 'regular mass-murderer', which implies it's against the norm). Gojyo and Hakkai did these together. Let's assume complete slaughter from everything we've seen to date and after, and let's assume the groups were probably of 40; divide by two if they split the kills evenly: 20. Let's assume Sanzo sends them on a mission once a month (may be high, may be low). For three years, that's approximately 720.
- Then they go on a trip which has lasted a year so far and which they are frequently being attacked by assassins -- so much so they make jokes about popularity tours and so on. Once in a while the assassins are individuals; more often they're roving bands of ten to sixty, depending. It's hard to say just how often they're attacked -- however, a filler episode in the anime (not manga canon) plays off this fact to guess that Sanzo's shot about a thousand people (in order to make a joke) and this seems, in context with what had been seen to date, to be perfectly reasonable. Hakkai tends to do about the same amount of fighting as Sanzo. Given that the anime probably was relying on the believability of them fighting that many, I'd say 1000 in the last year is pretty reasonable. Maybe more believable in anime than manga, but the manga also ellipses more time with just "We've been fighting a lot of enemies lately!" so.
That is approximately (very approximately): 3250 persons killed by Hakkai alone.
Over the course of about three years.
(For those who want to play with statistics, that means 1082.3/year average since he started killing. 90.3/month average. 22.6/week average. 3.2/day average. Mind, you can't really play with averages, since the majority is stacked at the very start, but. BUT. THE PURPOSE OF THE HAKKAI IS TO FLIP OUT AND KILL YOU.)
GG, Hakkai. GG.
(Am I missing anything? If so, do tell.)
ETA:
eoan: Looks more or less reasonable. Might be wary on the side of the missions from Sanzo being mostly about killing -- world was a bit more of a peaceful place back then, and the Sanbutsushin are better at the no-killing thing than Sanzo is, presumably. Being Buddha and all._diesmiling_: Good point. Mind, I'm thinking more that Hakkai and Gojyo would do things their own way, and both are completely comfortable with casual killing as a response to attacks by the time they get to go on the journey, which makes me think it's almost routine, but ... good point.eoan: In anime, at leat, it's routine enough that Gojyo doesn't hesitate to kill the youkai he was playing cards with when the wave hit Chang An.eoan: Instead of, say, beating some sens into him._diesmiling_: Manga too. The only time they don't kill is when they "don't feel like it", and even then they might. Manga's a bit more gruesome with it too -- his weapon's MESSY.eoan: But the anime doesn't really treat youkai as people. At least, Gensoumaden doesn't._diesmiling_: And our first introduction to Hakkai and Gojyo in the manga is, in fact, the scene that has them (and Goku) all saying the line: "We'll tell you if you survive" while attacking._diesmiling_: And Gojyo crushing someone's head in his hand.eoan: In the manga, though, by the time we see them killing it's pretty obvious that everything's gone to hell, isn't it?eoan: Heh, yeah, either way though, they're used to it._diesmiling_: [nods] at that point the minus wave had been going on for a year.eoan: It had, but has it spread to where they lived?_diesmiling_: Lemme check a geography thing, but by the time they meet Shunrei the demons went crazy "a year ago"._diesmiling_: Can't find the distance of the city with Houmei ("Town of Kouchin"eoan: It was a ten day's drive, though._diesmiling_: The time period specified THEN is "just a little while ago", mind.eoan: In distance.eoan: Which doesn't say much, given that we have no idea how fast they go. Rather, how slow._diesmiling_: Yeah. [nods] It's hard to say, but. Given that they're on the trip for a year and that gets them halfway to India..._diesmiling_: Which is the source of the problems...eoan: Pretty damn slow._diesmiling_: And we can assume the wave travelled somewhat quickly, given that IIRC the experiments also started about a year ago afaik?eoan: It's hard to tell. Yaone was picked up at least three years ago._diesmiling_: [nods] Valid.eoan: But that side of the timeline's always been pretty vague._diesmiling_: Yeah. Well, we may well drop off 200-500 kills off that count to estimate low, but.eoan: But that really doesn't make that much of an impact.eoan: Because Hakkai's crazy._diesmiling_: Absolutely.