How is being created for slavery not justification enough? Also an engineer ripped his head off when he meets one and on top of that they appear to have created a horrible xenovirus so that they could wipe out whole planets full of their creations in one go, does he need more justification to think they don't deserve to live? Engineers appear to be even more flawed than their experiments. Lets not forget that its made clear that David has been losing his marbles in the last 10 years on top of everything else.
Maybe you dont think its justification enough, but its enough for me.
Losing his marbles. Fair point. If it is the case. Perhaps Weyland programmed him to go bonkers once he is dead. Out of spiteful ness about his remark about him out living Weyland.
Getting his head ripped off. Good point can't dismiss that. He may be sitting with the same god complex that Weyland had.
I don't buy the slavery thing. Sorry. Weyland called him son in both movies or closest thing to a son he will ever have and when weyland says he lacks a sole David doesn't see this as negative IMHO . He is more like accepting in a reflecting and sad as "I know I don't have sole" I am lesser than you spiritually " kind of way. He knows he is product. But its not about slavery IMHO.
Why would he care about the engineers killing off mankind if he himself wants them dead?
Oh yes the pod cast also mentions that the novelisation has Walter checking out the planet before the crew sets down. So there was inspection proper inspection then.