Could you please explain the reasoning behind that? I am having a heck of a time wrapping my head around this one.
The killing blow is basically a lucky shot! It is a fluffy lucky shot. Let's say that you have a weapon that has killing blow. Let's use Tiq'Taq'To for the example. His mount has the killing blow ability. It means that when he flies by, the mount attacks. Once the mount hits, the mount flies up in the air and drops the target. This wounds on a 4-5 (on most To4 targets), but on a die roll of 6, the attack is just so good that it bypasses the armor (but not the ward save). Another example would be a challenge in which the killing blow would be like a one shot decapitation. Hope that helps. Clear as mud, right?
It's when it gets to Heroic Killing Blow that things start to get a little silly. Thankfully the only people who can use that one are Brettonian lords and Skulltaker.
Chakax doesn't have HKB... As far as I know, the only things that currently have heroic killing blow in the game are: Skulltaker Brettonian with virtue of heroism Tomb King with destroyer of eternities Necrosphinx Daemon slayers actually don't have it (oddly enough), and I can't think of one special character besides Skulltaker that does. I've been wrong before, but can anyone name another specific instance of HKB besides the ones above?
No instances of heroic killing blow... I can think of many cases where magic items have HKB - like effects, if you see what I mean. Vampire Frostblades, Lizardmen's Blade of Realities, Dark elf Venomsword, Wood elf Hagbane arrows (but they will never do anything >.>) all of these can cause an enemy to die on the spot, even though it's not technicaly heroic killing blow. Having said that, Skulltaker only gets his HKB in a challenge, so it's not much use to you against a hydra. The Venomsword allows the assassin that uses it only three attacks, so it seems unlikely to wound a T6 monster (let alone have him fail his saves), and Hagbane arrows are not only less likely to wound, but they also require a T6 monster to fail a toughness test, so not too common. The sphinx may have heroic killing blow, but only for one attack, so I fail to see the point exactly. That leaves Destroyer of Eternities and the Virtue of Heroism. All in all, I would say that we're going to have to wait for the new edition of army books before this becomes a very prevalent rule :/
In their army book, Skulltaker just has regular killing blow all the time, like the other bloodletters/ Khorne Heralds. Only in a challenge does it become heroic (and work on a 5+ instead of 6). Nothing in the FAQ seems to suggest differently.