After Julia refuses (or simply just can’t bring herself to do it) Barn kills Woodard. Magnus, What’s the big deal? What was so shocking about this? Barn’s killed before (so it's not his first murder) and Julia kills no one. Is it that it’s not a “bad guy” getting killed? (I ask curiously & with all due respect.)
dom-- I tend to forget that Barnabas takes the syringe from Julia and does it himself, too. That may be a matter of the show flinching from showing the bizarre and disturbing moral moment that they seem to have been leading up to, where perhaps Julia did (originally) do the injection.
The situation is so messy that as much as I want to let Julia totally off the hook, I can't. She prepared the injection. She met Barnabas there, not telling anyone else, not getting help. She kept silent afterwards. The consequences would have been extreme for Julia if she'd gone to the police (probably involving Barnabas killing her that night), but when you're about to help to kill your old friend, you do something. She was a co-conspirator.
If Barnabas had just shown up and throttled Dave on his own, none of this shock would have been there, except that for the audience, the bad guy wins. Everything you say is understandable and makes total sense, though. Julia didn't do it. Technically. Maybe it's the cold-bloodedness of it, maybe it's our putting ourselves in Julia's place, imagining being in a position you probably can't ever forgive yourself for. I'm not sure. It's certainly unexpected. A very original moral situation is thrown at us.