I'm inclined to rule that renfields aren't human anymore - for the simple reason that I don't think you can soulgaze them, because there's nothing left there. They don't have even a scrap of mind or free will left to them. As such, I'd rule that killing, transforming, etc., a renfield is never a law of magic violation (though using necromancy on one could be, for the same reasons that a corpse isn't human anymore, but necromancy on human remains is still prohibited.)
(By contrast, I would rule that killing a human in self defense nets you a lawbreaker power - even if the Council decides not to punish you for it. I'd also be strongly inclined to make such an event count as a major milestone, with +1 refresh all around.)
Of course, that's a separate thing from how the Council views it. If the PCs can prove that those were renfields that attacked them - and that they knew at the time that's what was up - they should be on solid ground to avoid more than a warning. If the PCs are relying on the "self defense is okay" argument, well, that seems to be a leniency that the Council only offers to its own - remember that Harry was almost executed for self-defense, and the only reason he skated out is that someone was willing to take up the Doom and take him on as an apprentice.
And in a case where the PCs already have a black mark against them... I can't see the Council reacting with anything other than kill first, ask questions never. Unless, as someone suggested, somebody (either within the Council or a sufficiently powerful outside force like a Freeholding Lord) decides to stick their neck out and intercede on the PCs behalf.