Human Beings are composed of three parts, body soul and spirit this includes those who would deny the existence of the latter two. In my opinion, I believe shock atheists as you call them offend people purposefully because deep down their spiritual nature is restless. Man literally cannot live by the mere material things of the world, they don't sustain him because his soul still thirsts after things.
You see it everyday in the world whether from the rich on down to the lowest of low, people who live solely for the material things have a miserable existence. They are never satisfied, content with what they have even. This is why you see a lot of drug abuse, ridiculous purchases made for the sake of it, and brutal activities. No matter what a man does he cannot quiet his soul even if he denies his existence.
This is why I believe that shock atheists do the things they do. Their spirit is so disquieted, especially in the presence of other spiritual people, no matter what religion, but especially Christians. It is an uncomfortable feeling so in order to alleviate it they must hush the spirituality of others, and take joy in causing doubt where they can.
Offending people is akin to raising a brick wall. They equate atheism to intelligence and spirituality to ignorance. If you can't see it, touch, weigh, measure, hear then it can't possibly exist. Even though through physics we learn that there is more that exists than what a man can measure. However, if you look throughout history of man, many intelligent people were also very spiritual. I won't say Christian , but they acknowledged their spiritual man.
If an atheist were truly what they believe, it wouldn't matter if someone was Christian, buddhist, muslim, or whatever because it doesn't matter. You can rest comfortably in your position because you know at the end of it all it means nothing, everyone fades to black. They can't though because as ludicrous and un-quantifiable and undefinable as it is, the spiritual man still exists and that uncomfortable feeling is because that spirit knows at the end of the mortal life it must return to God and give an account.
Shock atheist is basically mr grinch, who wants to squelch everyone's spirituality because they cannot face their's. My apologies for the long post, I meant to keep it quick with a simple analogy.