Weyland, talks to David though the dream machine, tells him to go further in finding out about the Space Jockeys, David then tricks the douche into saying he would do anything for more information about the SJ.
Then David tests the goo on him to see what happens. When this fails he then finds out about the cunt squid and decides to let that come to term for the same end of having something to experiment on.
While it looks like David takes joy in these actions it's actually just a way of his programming making him carry out orders ;
http://www.youtube.com/watch?v=S7YK2uKxil8
this vid shows that liking Laurence of Arabia was something David was made to do, showing his joy though servitude to his programming.
Most directors would have made it a duel role, they probably had rewrites and just decided to use Guy to the full extent of his contract rather then hire yet another actor.