I do not think that's actually Dean. I think it's Michael pretending to be Dean to get closer to the group and harm them in some way. :/ As you pointed out, Gadreel was able to hide in Sam when he wanted to. Maybe Michael is doing the same thing... or maybe he's still 100% in control and just impersonating Dean. :/

And I also think that Nick killed his own family. I think maybe he's been suffering from blackouts like the mini one he had with Cas when he snapped his fingers for a long time, even pre-Lucifer... if he did kill his family, I don't think he realizes that he did. I think maybe the neighbor witness initially said he saw a man leaving the house because he realized Nick was the prime suspect and was misguidedly trying to be nice/supportive, but then he recanted it because he really did not see anyone leave the house... because no one did, because Nick did it himself.

These details coming out probably set off warning signals in Nick's subconscious, a little too close to the truth his mind has buried, a little too upsetting, and that's why he killed his neighbor. Back in the bunker later he probably won't even remember doing it. :/