Shaw chose the destination.
And once David got there he prioritized his own survival over everyone else's.
That's the surface level explanation.
The other reasons are up to speculation (and dont really feel necessary to delve into).
David could've
a) diverted the course of the Juggernaut to some other primitive planet
and settled down in peace and eternal boredom.
b) Taken the juggernaut to Earth? Where David would probably be destroyed and go back to slavery soon enough.
(The 1st Juggernaut was taken down fairly easily by the Prometheus.. and that human ship didnt even have military equipment.)
C) Or does David press the "button" for instant genocide and gain himself a whole new planet full of specimens? We know he's read information about "Paradise" before Shaw decides to go there... David probably knows about the few defenses of the planet, etc... The Engineers arent shown to have any "natural predators" in the known universe and They arent expecting one of their ships to suddenly be used against them...
hmmm decisions, decisions..
What would a
pervertedbisexualimpotentgeniushellbentonrevengeagainstmankind-robot
do in such a situation?
I wish the whole David/Shaw story had been expanded in a novel somewhere but alas.
