D&D does actually deal with remarkably clear lines between Good and Evil. Demons, devils and undead are all clearly in the evil category, as are most of the monsters you need to kill.
Of course you can play it with a lot more moral ambiguity than that, but out of the box, D&D is very suitable for a black & white world view.
I once worked for some people with a similar belief system to that above. A big part of it was that there were only two powers in the world - Christ & God vs. Satan. ANY power that was not explicitly that of Christ was implicitly the devil, regardless of how good it sounded. Harry Potter for example, was clearly an agent of Satan.
Put the party into a situation where they could ally with a profoundly evil power to achieve a great good. Or maybe give them a powerful magical item that they discover is slowly corrupting them as they start wielding it for the right reasons...
I wish more of our current crop of politicians had played RPGs that way...
Of course you can play it with a lot more moral ambiguity than that, but out of the box, D&D is very suitable for a black & white world view.