I think Nick didn't kill Donna, because he hasn't killed many (maybe any?) people who are unrelated to his quest. You're making me think about what he has done, which is hard, because I too tend to zone when Nick is on screen (don't ever ask me about why Maggie does anything I have no clue). I know he killed the people he thought knew something about the night his family was killed, the neighbor, the cop, (did he kill the newspaper reporter, I don't think so but again I zoned). I'm pretty sure he resisted killing some woman he ran into on the street, just sent her off. Since he saw Donna as uninvolved and just doing her job (like the maybe alive reporter) he didn't see a reason to kill her.

I do think that the vessel for Lucifer does influence Lucifer's personality. The Rick Springfield Lucifer was very different from Nick Lucifer (wilder, angrier, more frightening, more dangerous - I like that Lucifer) and President/Lucifer was different as well, horny not as whiny. Sam/Lucifer in The End was calm and didn't whine and was very sure of himself. We haven't seen enough of different Archangel vessels to tell if it's just Lucifer or if Michael gets influenced as well.

This was a pretty good episode. Davy Perez is one of the newer writers and I've been happy with all three.