There are 3 possible motives for David.
He wanted to test the black-goo to " try harder ".
David was receiving orders from Weyland as he told "Try Harder!".
The scene where David spikes Charlie's drink happens after the 'try harder' scene (with Vickers).
David goes to Holloway, asks him how far he is willing to go to find his answers, and Holloway replies he would do whatever it takes .
David takes this as an invitation to experiment with the goo on Holloway , ultimately at Weyland's request.
David is simply following orders.
So my guess is that, he probably wanted to test it , just in case it turned out to be what Weyland was looking for, an immortality potion or something He wanted to kill the crew to gain his freedom .
David starts to hate the humans.
Note how antagonistic Holloway had a jerk towards David.
When Holloway asking David why he's wearing a suit as much to say '
you're not one of us, why are you bothering to try?'
He wanted to kill Dr. Holloway to remove him as competition for Dr.
Shaw
He seemed to pay more attention to her from the start by looking in on her dreams and taking special care of her when she awoke from cryo.
I think when he tried to put her back into cryo he was trying to do what was best for her.
I also thought that, at the end, whenhe said he was afraid she had been killed that he was beinggenuine .
David:
I was scared you were dead.
Shaw: you know nothing about being scared, david.
movies.stackexchange.com/users/556/Vijin Paulraj
David has multiple motivations: to help Weyland achieve Weyland's goal, which is to contact engineers so engineers can help Weyland cheat death.
After all David is a computer with a programmed purpose: serving Weyland.("Try harder" is Weyland telling David to keep looking for a living engineer, which David then does successfully.) to reveal how humans were not special but instead -- like David -- were created by other intelligent beings.
This is the irony underlying Holloway's insulting David.
movies.stackexchange.com/users/763/Shiz Z.
In the very beginning of the film David's hair style is different, then we see him watching the film while he is coloring the roots of his hair to match Peter's, and then he combs it so that he has the same hair style.
He later repeats the phrase "The trick, William Potter, is not minding that it hurts".
I think David repeats that line two or three times.
I took this to imply that David's natural appearance is different including his hair color.
Is this sequence connected to David's fascination with the Engineers.
Absolutely, because this scene defines a repeating theme that David tents to overly obsess over things related to the human condition.
I think you have to watch Lawrence of Arabia to understand what was being implied by that scene.
Why did David infect the Doctor?
David is seen reporting to Peter Weyland that the Engineers are all dead, and he then explains that Peter Weyland ordered him to "try harder" to discover what happen to them.
Did Peter tell David to infect a crew member, or did David conceive this idea on his own?
That isn't made clear in the film.
Later in the film, Peter Weyland is told to take off his helmet and Elizabeth protests saying that the infection that killed the Doctor could be air born.
Peter Weyland appears to not know that the Doctor is dead, and David just gives Elizabeth a look.
What was his motivation?
David says "big things have small beginnings" and then later while talking to the Doctor.
He says "if you could meet your creator, what would you say?".
I really feel that David understood that the dark fluid was a virus, and that it created life.
In that scene David expresses a fascination with meeting ones creator, and also his disapproval of humans.
I think he gave it to the Doctor, so he could destroy his creator and turn himself into a creator at the same time.
While answering the question, what does this black stuff really do?
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Here's my biggest problem with it: David knows absolutely nothing about the black goo.
What would make him think that it could create, change, preserve, whatever, human life?
I mean, if you happened to come across a bunch of black goo would your first thought be to feed it to some animals.
Even if it contains a bunch of nutrients, eating an egg doesn't cause a chicken to start growing in my body.
So he just comes up with this alien virus/embryo idea out of nowhere.
Not only that, but they have an awesome lab there, why doesn't he just use that to run some experiments on it?
Why doesn't he work with the other scientists to find out more about it?
Surely that would aid the old man just as much if not more than david working alone.
Even if you make the argument that he is just following orders, that just shifts the blame from him to the old man.
It doesn't really change the fact that his motives make no sense whatsoever.
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My 2 Strongest theories about this are: He turned out to hate the humans due to the actions of Dr.
Holloway .
To me, since the first time David and Holloway met... I always had the gut feeling that David was going to do something to Holloway sooner or later.
I mean, Holloway was being such an ass towards David!
Plus, not only that..
but.. to me, it seemed as if he knew that the black goo was dangerous.
One reason why I believe he knew is that he didn't inform the crew about taking that black goo back with him.
He has a thing for Shaw.
If you think about it, what Vijin said is true.
And, I do not want to type all about it when the informations already up here, lol.
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For me, perhaps the most interesting person aboard the Prometheus was the android, David. I'm still confused by his seeming fascination with the movie,  Lawrence of Arabia . Although I couldn't make notes during the film, I believe that he quoted Peter O'toole's character, Lawrence, on more than one occasion. Was he attempting to attain a similar achievement with the Engineers as Lawrence did with the Arabs? Why did he infect the Crew leader? What was his motivation?
What was David's motivation in Prometheus?
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<plot-explanation><character><prometheus>
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David is a computer with a programmed purpose: serving Weyland.is motivation is  to help Weyland achieve Weyland's goal, which is to contact engineers so engineers can help Weyland cheat death. David has multiple motivations:
David is a computer with a programmed purpose: serving Weyland. He is designed to contact engineers so engineers can help Weylands cheat death. David is also designed to reveal how humans were not special but instead -- like David -- were created by other intelligent beings.