I put on my thinking cap and wracked my brain to come up with these examples.
Table of known deaths by increasing count..
Dierdre Page/Phillip Henry
0.25 (Each) For their share of responsibility in death of Randall.
Giles, Ethan, Dierdre, Phillip & Randall were members of a
group that was getting 'high' on Eyghon possession. When
Randall became irreversibly possessed they tried to exorcise
him, but it resulted in his death.
0.25+ Randall (0.25).
Cast two spells which caused an unknown number of deaths.
0.5 Shot Billy Blim. He looked human, but was part demon.
0.75 Randall (0.25)
Ben (the human form of Glory, a Hell God - 0.5).
1 Jonathan Levinson.
1 Had Darla sired by Drusilla, after Angel refused to sire her.
1 Killed Holtz in a way that implicated Angel (at his request).
1 Killed Professor Oliver Seidel - Fred's former professor.
1 Murdered her son Dennis (later Cordy's ghost).
1 Infects Fred so that Illyria might emerge in her (dead) shell.
2 Katrina Silber his ex-girlfriend (violent though accidental).
Tried to kill Buffy Summers, but got Tara McLean instead.
2 Mostly from fear inspired out of terror. Once she had learned
to control her power (telekinesis), she chose not to kill.
2 Deputy Mayor Allan Finch (accidental in heat of battle).
Professor Lester Worth. A geologist/volcanologist. Cold..
2 Warren Mears (revenge).
Rack (for power, indifferent - "trust me, it's an improvement").
Dana (potential slayer)
? Dana was raped & traumatised as a child and institutionalized.
When the potential slayers were activated by Willow, she caused
an unknown number of human deaths.
10+ The 'basement full' of vampires he sired while ensouled.
? Summoned the Quellar demon to kill the insane people.
Caleb (The Preacher)
? - A misoginystic psychopath, who left a trail of corpses
(at bare minimum 2) before becoming the vessel of The First
and going on to kill lots of watchers and potential slayers.
? Before they were sired, they "made their reputation by
massacring an entire Mexican village in 1886."
35+ - Sam Lawson. A lieutenant in WW2.
- Was responsible for (he chose not to stop it) a Thesula demon
chowing down on the residents of an hotel. (at least 20)
- A doctor that was stalking his own patient.
- 13: 12 Wolfram & Hart attorneys + the wife of one that he
allowed to die.
- Drogan - though note he is described as a 'Mystic Warrior'
rather than explicitly human.
So the 'winner' would seem to be one of the following:
- Lyle Gorch
- Tector Gorch
It is hard to delineate between them given the numbers are so nebulous, and cannot be pinned down further.
Notable side mentions. Excluded from the list above for the reasons given.
Professor Maggie Walsh
0 Tried (but failed) to kill Buffy by setting a trap with demons.
Billy "Ford" Fordham
0 Deal with Spike to sire him in exchange for Buffy'n'snacks.
Interestingly, Spike kept his side of the deal even after it
all turned out for the worse (for him) with no deaths.
Daniel "Oz" Osbourne
0/1 But both he and the deceased were in Werewolf form at the time.
2 Lilah, and a young girl as sacrifice. Though it is not clear
if she had a soul (she was possessed at the time).