Rather than focusing on the wording of the rule, I'd also interpret the rule teleologically, i.e. considering the intention of the rule, e.g. "can it really be true that they wanted the card to be this powerful/weak?" This is how I tend to play my games, and it's carried me pretty well so far.
So, whenever I come by a rule that might net one player some 20 VPs in one turn, that's when I become very suspicious. However, I have to admit that I have been surprised by FAQs before, so I can't say what FFG's final ruling will be on this matter.
Furthermore, I have to admit, the old set of upgrade cards was horribly imbalanced (for example, who'd take the old Bloodletter upgrade over anything else Khorne has to offer - or who would even consider taking the Slaanesh Warrior upgrade, ever?), so there is a chance that the expansion deck is like you said and "as intended" that way, and they actually did want Khorne to have this powerful an option. If that's the case, I'm sorely disappointed in FFG, because this was one of the games that struck me because it seemed so finely tuned when it comes to balance.
MR Suplex said:
Also, you seem to be mixing upgrade cards from the base set and the expansion, since the Khorne cultist upgrade you are referring to is from the base game.
True this. I had already written my response and then I started thinking whether there was another card could also consider taking. That's where I mixed the two sets, in a moment's lapse (imbued upon me by Tzeentch, no doubt). This doesn't really affect what I said about the Bloodletter upgrade, however. On the contrary, it reinforces the idea that there are very few actual options to the new Bloodletter upgrade.