If we assume that David sees the same qualities in it that Ash later does, I don't think it's the simple fact that it can "wipe out the human race in a terrifying way". There are any number of simpler things, like chemical or nuclear weapons that could do that just as effectively, and probably more efficiently.
As Ash says, what he admires is the fact that it doesn't suffer from "conscience, remorse, or delusions of morality", which are traits that he despises in humanity (because of their delusional status) and perhaps resents for them foisting on himself, as a synthetic. With David there may also be the sense that, as his creations, they are inherently more perfect than anything a flawed humanity could hope to create. The irony there is that it reveals his own imperfection, and perhaps deep-seated insecurity and self-loathing as a result.