Hmmm, perhaps SK is also an apostle. The Angel woman asked Guts if he was going to suffer or sacrifice something, like he (SK) did. So, SK did sacrifice someone in order to gain his form, meaning he might be an apostle. And he WAS branded as sacrifice, told when he mentioned to have once survived an Eclipse as well.
However, an Eclipse is the birth of an Angel. And there are only 5 including Griffith. I suppose the Angel that "sacrificed" him was the woman. Though nothing so far has hinted it.
Anyway, if Guts sacrifices someone that hasn't been branded yet, to be able to kill the angels, it might be worthy. Remember that he can save this person from being eaten, just like he was saved by the SK. I can even imagine the Old witch having a brand as well, and that being the reason why Schierke knows about it. And the reason they both knew how to seal brands. The witch would have been the SK sacrifice, although he never allowed her to be taken as such, using the powers he gained from it.