Why did the vampire Marius ... spoiler

99% sure he showed up to turn him, rather than just kill.

Anyway, I'd say Marius was looking for a new companion, and Talamascans are pretty good candidates to turn. They already know a lot more about that world than regular people, and usually respect the boundaries that come with it. In short, they're just more in tune with the vampire world, intellectually speaking, so turning a Talamascan is a no-brainer.

Just my 2 cents.

Why do you think he killed or turned him? Did I miss a scene?

I don't know if he was there to turn him. I do think that Marius was there for a visit -- maybe to mess with David's head or to talk about his paintings. After all, David said that Marius was his obsession and perhaps Marius was following up with one of his biggest fans. Either way, I liked Marius' character and they kind of left it open to interpretation.

David Talbot is a long time partner and confidant to many people [read; vampires] in the books.

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I hadn't read The Tale of the Body Thief when I watched the movie Queen of the Damned back when it was in theaters. Even though I hadn't read this book, I imagined what (probably) most of the book readers had... that Marius was there to turn David, not kill him. The look on Marius' face before entering the building was filled with mischief and playfulness. With how Marius is presented in the movie, he wouldn't have killed David. He was too busy having fun with David's obsession with him. David was the right kind of person Marius could see as a companion. Also, Marius wasn't like Lestat (for the movie at least), he most likely would have turned David even though David turned down Jesse's offer.

BOOK SPOILER
The reason why most imagine that Marius was there to turn David is because in the 4th book David is turned into a vampire... only not in his original body. David did turn down the offer of becoming a vampire, if I remember correctly but could be very wrong, because he was old--much older than in the movie--and didn't want to live that way. Here's a link to an okay wikia source summarizing David...
http://vampirechronicles.wikia.com/wiki/David_Talbot

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I don't think marius killed him. I think he just wanted to talk