You can do this with either guard, Junius or Bibius. I've posted video of me doing just this against Junius
The point is that Junius is the trigger NPC for the quest; I suspect (and will go test) that killing Bibius won't impact anything at all.
[Edit]
If you kill Junius first, then Bibius (in separate combats), you get no inter-party dialogue (the golden ! = "Do you think we should have..." leading to +1 in traits); however, killing Bibius first, then Junius leads to it triggering.
Which is a bit odd.
On the other hand, if you do the same to the NPCs on the beach, on taking damage they spam you with a dialogue threat, but
do not start combat, nor do they loose attitude, and are quite happy to have chats even after loosing 90% of health. On dying, the inter-party discussion about killing innocents does trigger.
So, basically - looks like DoS doesn't like the quest trigger NPC dying first.
