Culling Blade's kill threshold is supposed to be 250/325/400 (300/425/550 with Scepter).

But this is not the case. Instead, it is 249/324/399 (299/424/549 with scepter), off by 1 on each level, with and without scepter. Looks like the codes use only "less than", instead of "equal to or less than".

Here you can see how a level 3 Culling Blade (kill threshold 400) cannot kill a full hp level 3 Beastmaster Boar (400 health)