I am trying to understand this vote and these advantages. I totally get the steal-a-vote play. The nullifier, though, I question. Did they need to use that? Or could they have just anticipated Dan played an idol for himself, then vote for someone else like Kara? The way it played out and the actual votes that were cast lead me to believe that Alec, Alison, and Kara were on board with flipping, and the Davids knew it. They knew only Dan, Angelina, and Mike would have votes for Christian, so they threw a couple votes to Angelina as insurance, they didn't want to take out Mike as he's an easy vote later on, and they want Dan out now. The edit looked like Alec, Alison, and Kara only flipped because they saw Nick steal a vote, which I don't know if that's truly accurate. And the reason they stole Alison's vote is to take the scent off of Dan, and the nullifier was just an insurance policy in case Dan played it on himself. So many moving pieces here, and I don't know if what I typed is understandable... I was trying to play out the scenario in my head.
My initial thought was the same, why not steal Alison's vote as a decoy and then do the vote against a Kara, which the Goliaths would have been unlikely to predict.
Maybe they just wanted 100% certainty, which is what they got with playing the idol nullifier on Dan. Considering how hard it would be to play that at the right time on the right person, I can't fault it's use now. Dan is a threat in the game, and getting him out is wise.
They also seemed to be skeptical about the Goliath's flipping, so again I see where playing it safe was a good choice.
This was a key moment of the game, and the David's played it well. They worked together and shared information & advantages in a smart way. Advantages are often held in secret to be used as an individual, but using them together has paid off, twice in a row.