by planting doubt where maybe there was none before..
Doubt is not the end of knowledge; it is the beginning of it. You can't "plant doubt". They doubt, or they do not. It's hardly a noble act to shield someone from fact lest they doubt something.
what's to be gained by stomping in and destroying their safe 'happy ever after' little universe?
But we're back to the assertion from the OP that you couldn't support - that atheism means the end of happiness.
ignorance really can be bliss...
You don't know that it was for that guy.