If you've got the glass ball that lets you see they don't turn into a world full of Reavers or Breen or Imperial space nazis, that can justify intervention, assuming you are willing to play God with history. But revealing the giant metal bird to them at an impressionable time is monstrous.
Kirk might be condemning them to millions of years of religious strife, and I don't know that is better than letting life evolve anew there after the volcano plays out (assuming it really killed everyone, which sounds a little O-T-T ... is the volcano going to cause non-nuclear winter for the whole world?)
I disagree. You agree that the Prime Directive is ridiculous. I say, not because it's unenforceable, but because it's a silly principle. What does "destiny" mean anyway ? I agree you shouldn't make contact with non-warp-capable planets, but between letting all life on a world die, I think that was a minor offense. Calling it "monstrous" is a bloated hyperbole, at best.
Would you have let them die ? Not me.